Sassy is a Himalayan cat owned by a young girl named Hope who has a very close bond with Sassy. She has a sarcastic nature which makes her truly live up to her name. Her pet comrades are Shadow, a wise, old golden retriever and Chance, a young, immature American bulldog. She has been living under the same roof with the two dogs for quite some time. She even shares a sibling relationship with both of them. She gets along with Shadow very well with hardly any arguments. When it comes to Chance though, the two would often clash and have disagreements about their breed and species differences and which one is best, with Sassy always insulting Chance with the slogan of "cats rule and dogs drool", throughout the first and second movie, Chance turns this back on Sassy in some cases with rephrasing it as "dogs rule and cats drool" even though at one point she isn't around to hear it. Despite this, they have shown that they do care about each other as they are willing to help them whenever one of them is in trouble.
In the film, Sassy is checking on Chance as he is chewing up some clothes while Hope is outside attending the wedding of Bob and Laura with Jamie and Peter, along with Shadow. When Chance accidentally swallows a button, Sassy sarcastically retorts that "too bad it wasn't a zipper". Taking offense to that, Chance threatens her to leave or else, but when Sassy calls his bluff and insults him, Chance attacks and Sassy flees, their commotion drawing attention from the wedding guests outside, including Shadow, but Peter gets him to stand down.
Later, Sassy continues to try and prove to Chance that her motto of "cats rule and dogs drool" is legit when she manages to pull something off that Chance tries to copy, but fails spectacularly on, which is begging for food. Sometime after the wedding reception is over, Sassy is with Hope on the seesaw when Chance sees an opportunity to humiliate Sassy by scaring her onto the other side of the seesaw before he pounces on the end she was just on, sending her catapulting into the sandbox or "big litter box" as she calls it, to Hope's shock and disbelief while Jamie and Peter laugh.
- “That's why they call it the "cat-a-pult"."
"You've got a lot to learn, pup.”
- ―Shadow not impressed with Chance's stunt
While Hope takes a freaking out Sassy inside to complain to Laura about what Chance did, Chance gets his karmic payback when the wedding cake he ate earlier makes an unexpected return back up his throat, to Jamie and Peter's disgust.
Sometime later, the Seavers are taking the pets to leave with their friend Kate for the time being while Bob is busy settling into his new job back in San Francisco. While there, one morning, Kate accidentally steps on Sassy's tail while serving breakfast to the pets, and not even offering Sassy her meal "warmed up" convinces her to come back down because of what happened, leaving Sassy to watch when Chance decides to dig into her food since she apparently did not want it. Later, Kate takes off to go on a cattle drive, leaving the pets to be cared for by her friend Frank. However, Shadow suspects something is wrong when it is clear that the Seavers have not returned as soon as he expected, and after telling off both Chance and Sassy when they think the Seavers abandoned them, Shadow decides he's going home and leaps over the fence to take off to return to Peter. Although Sassy is initially reluctant to go, she finally concedes and joins Shadow, with Chance joining soon after when he gets spooked by the turkey who frightened him earlier and wanted nothing else to do with it. Together, the three pets set off across the Sierra Nevada mountains to return to the Seavers. When Kate and Frank find out, they notify the U.S. Forest Rangers to keep an eye out for them, and when the Seavers learn of what's happened, Peter is suggested to put up lost animal flyers by the San Francisco Police Department in the hopes that someone would find the pets.
As Sassy, Shadow, and Chance make their way through the forests of the Sierra Nevada mountains, after a harrowing escape from a large brown bear after Chance spooked off a pair of younger black bears, the pets reach a river that Shadow knows they have to cross to get home. While Shadow and Chance ford the river no problem, Sassy has no intention of getting herself wet to follow them and instead finds a set of rocks to use as stepping stones across the river, all the while believing she is smarter than Shadow and Chance as she sings her motto of "cats rule and dogs drool" as she does. However, as she reaches the other side and starts to cross a rotted tree trunk to reach safety, the trunk collapses underneath her, dropping her into the river, sweeping her downriver towards a waterfall. As Sassy meows and cries for help, Shadow tries to get to her in time to save her, but is too late, as Sassy is swept over the falls with a screaming, screeching yowl. Shadow and Chance presume her dead and continue onward with heavy hearts over their loss.
Unknown to them, Sassy survived, and ends up being retrieved from the river further downstream by a local hermit named Quentin, who takes her home to his cabin to nurse back to health. Sassy makes a full recovery thanks to Quentin, and when she hears Shadow and Chance sometime later barking in the distance, she immediately takes off to reunite with them as Quentin watches her go, wishing her good luck and to stay clear of the river. Sassy soon reunites with Chance and Shadow, and the three continue their journey. However, Chance soon tries to befriend a porcupine, and annoyed by Chance's persistence with wanting to talk to him, the porcupine swats Chance with his tail, leaving several quills lodged in the side of Chance's muzzle that even Sassy can't pull out without hurting Chance in the process. Thus, Chance is forced to continue on with the quills stuck where they are. Soon, they encounter a lost little girl named Molly, and Chance and Shadow, following their instincts in being man's best friend and not wanting to leave her, stay with her throughout the night, with Sassy joining them. Come morning, Shadow hears the search party looking for Molly, and goes off to lead them back to Molly while Sassy stays with Molly to keep her warm with her fur. Once Shadow locates the search party of Molly's parents, rangers, and rescue team, he leads them back to Molly, allowing her to reunite with her family and get medical attention. The rangers then recognize Sassy, Shadow, and Chance from the lost animal flyers they received from the Seavers, and take them back to the local animal shelter where they could be reunited with their owners. However, once there, Chance mistakes it for the pound, and having had some bad experiences there in the past, he convinces Sassy to flee into the woods while he and Shadow are taken inside, with Shadow placed in a cage, and Chance has taken to have the porcupine quills removed by the shelter's medical staff. After Sassy is able to force the shelter's security guard into giving up looking for her, she sneaks back into the shelter to find Shadow, being offered several catcalls by the other dogs, before finding Shadow's cage and letting him out. After finding Chance in the doctor's office, because the guard was drawn to their location thanks to the excessive barking by the other pound dogs, Sassy used herself as a distraction to the security officer in order to buy Shadow enough time to bust Chance out, which he does after all the quills are extracted. Sassy ends up captured by the security guard in the process, though she, fortunately, was saved by both Chance and Shadow before they make their escape back into the woods and resume the journey home, totally unaware of the Seavers being on their way at that very moment to come to pick them up.
They soon reach where they can see the city in the distance and know they are almost home, but have one more obstacle left to cross: the local rail yard. After narrowly avoiding a diesel locomotive as it made its way through the yard with its freight train, tragedy strikes when a muddy sinkhole that was boarded over gives way under Shadow's weight, dropping him into the muddy pit below. As Sassy and Chance look down and see if Shadow's alright, they encourage him to try and climb out, but despite his best attempts, Shadow apparently concedes defeat and lies down, telling Sassy and Chance to go on without him. While Sassy remains on the surface and looks down, unsure of what to do now, Chance carefully makes his own way into the hole to tell off Shadow for giving up when they are so close to home and to give it another shot, but Shadow has apparently accepted he's too old and tells Chance to go on without him. Chance can only look up at Sassy for help, but Sassy is unsure of what to do now herself, before Chance looks back down at Shadow and again refuses to leave without him.
However, sometime later, it is apparent that Chance and Sassy did made the heavy decision to leave Shadow behind and continue home without him, as Chance soon crests the hill to reunite with Jamie, and Sassy shortly thereafter to reunite with Hope. Peter calls out to Shadow, but when he doesn't initially show, Peter accepts that Shadow was too old to make it and goes to head inside, only for Shadow, having found a second wind, to soon appear himself, and reunite with his boy. Together, the pets and their owners head inside for dinner, happily reunited.
- “Dogs rule and cats drool!”
- ―Chance turning Sassy's own motto back on her
In the sequel, the Seavers are preparing to go to Canada on a camping trip, but Sassy is having some fearful thoughts about it when she tells Chance that she saw suitcases and that means the family is leaving. When she hears Bob coming downstairs with an armload of clothes and mentions the car, Sassy scurries up the stairs, but in the process causes Bob to lose his balance and fling the clothes on top of Chance. Hope arrives to see if Sassy is alright before laughing at Chance's predicament as Sassy also has some fun at Chance's expense. Later, after overhearing Jamie tell Chance to stay home while he goes to his baseball practice, upset with how Chance has been behaving lately and not understanding why, Sassy sarcastically tells Chance that Jamie is probably going to send him back to the pound as punishment, and she won't miss him if that happens, laughing about it, leading Chance to send the pipe Sassy is laying in rolling down the driveway until it bumps into the family SUV. After telling off Sassy with saying his own version of her motto, but with "dogs" and "cats" switched, Chance takes off after Jamie, while Sassy stumbles out of the pipe dizzy, swearing that when she recovers, Chance is going to pay for this.
When Chance returns later with Jamie, Sassy is already in the car with Shadow, and Jamie snaps That Chance made him miss the game as he tells his parents the reason for why he and Chance are late getting home so they can still make their flight at San Francisco International Airport, leaving Chance to think Sassy was right about Jamie sending him back to the pound as punishment. As they head for the airport, Chance has a little fun at Sassy's expense...
- “Why do they shove those little, green kibbles down our throats? If you're smart like me you'll get rid of it."
"Get rid of it? How?"
"Easy, you just work up a nice sized hairball, and you think of something...disgusting. Like a big...fluffy...cat!"
- ―Sassy not amused by Chance's thoughts to induce him to throw up
Once at the airport, Sassy voices her concern to Hope about the animal kennel she's being put in for transfer to the plane, but Chance is the more vocal, as he's worried about Jamie being furious with him for disobeying his orders, especially when Jamie sarcastically says goodbye to him as the animals are taken to be loaded onto the flight. As they are taken across the tarmac, Chance notices the airport workers in their hazmat suits and immediately has his worst fears come to the surface in thinking that Jamie has indeed sent him back to the pound as punishment for his earlier actions. Although Shadow and Sassy try to stop him, Chance is able to knock his kennel off the baggage cart to split open being dragged in two opposite directions and free him to run back to find Jamie and apologize. Sassy is able to undo the locks on her own kennel and Shadow's so they can go after him, only to find the Seavers no longer in the terminal, forcing them to head back through the baggage area to the tarmac to try and find them there. Just as they try to figure out which plane the Seavers are on, Chance spots the airport's security forces mobilizing to recover them and try and get them back on their flight or the next one to Canada to reunite with the Seavers. Shadow soon picks out the plane the Seavers are on, and with the security team on their tails, the pets head for the runway. However, just before the plane can run them over, it reaches takeoff speed and lifts off, heading for Canada with the Seavers aboard, but not the pets. With the security team on their tail still, the pets head for the airport's security perimeter fence to escape.
- “If we weren't going to the bad place before...We're definitely going there now!”
- ―Sassy as the airport security team closes in to recover them
While Sassy climbs the fence and squeezes between the barbed wire on top and the chain link to escape, Shadow digs under and is able to squeeze through, but Chance gets his collar snagged on the bottom of the fence and struggles a bit before the fence finally pops loose and he is able to escape as well, leaving the security team to watch in disbelief at having been unable to recover the pets as they flee into the distance.
Once a safe distance away from the airport, Sassy voices her concern to Shadow about the plane leaving them behind and what to do now. Shadow initially thinks the Seavers don't know they are missing, but Sassy is quick to pin blame on Chance since it was his paranoia that has gotten them lost again. Shadow then states that, like it or not, they have to find their way home again. Chance doesn't want to, but Sassy is more than willing to follow Shadow again as they set off, with Chance reluctantly joining them shortly afterward. In Canada, when word of what happened reaches the Seavers, Hope immediately wants to return home to find Sassy, but Bob offers to head back alone first thing in the morning while the kids enjoy their trip with Laura, but with Peter's insistence that they won't be able to enjoy their trip knowing the animals are lost in San Francisco and Hope's plea, Bob concedes and agrees that they will all return home as soon as they can.
Meanwhile, as Chance, Shadow, and Sassy make their way through San Francisco to find the Golden Gate Bridge, which is what they need to find to get home since they crossed it to reach the airport earlier, they soon enter Chinatown and stop in front of a fish restaurant where Sassy tries to get some free food but is quickly shooed off by the owner when he notices her eyeing his merchandise. She later spots a young boy named Tucker and his kitten Tiger on the front porch of their house and tries to sweet-talk her way into getting some food from him, but when Chance tries to copy her, he scares Tucker back inside as his mother tells Chance to go away before slamming the door, leaving Sassy to reprimand Chance for screwing her over.
Later, they end up running afoul of Ashcan and Pete, with Ashcan attempting to eat Sassy, only for Shadow to come to her defense and attack Ashcan while Chance dealt with Pete. However, it is then that Sassy spots what she thinks is reinforcements for Ashcan and Pete, but it is really Riley and his crew, showing up to kick Ashcan and Pete off their territory, with Sassy being able to knock a flowerpot over onto Pete's head before the two bullies retreat, and Sassy and Shadow join up with Riley and his crew as they look for Chance, who took off earlier when he thought Riley's gang were Ashcan's reinforcements and tried to decoy them into following him, but it didn't work. Riley sends Delilah to find Chance, while the rest of them avoid a run-in with the "Blood Red Van" before Shadow and Sassy decide to try and find the Golden Gate on their own when Riley refuses due to his own fear of humans. However, after Shadow and Sassy unknowingly avoid another encounter with Ashcan and Pete due to Pete's own stupidity, they find Tucker's house now fully engulfed in flames, ignited by Jack and Ralph from the "Blood Red Van" when Jack tossed his still-smoldering cigar into the dry grass next to the house, setting it ablaze. After hearing Tucker's parents pleading to the San Francisco Fire Department battalion chief to send someone inside to find Tucker, as he's still trapped inside, Shadow decides to go in to find Tucker himself, to Sassy's disbelief. Realizing she has to do something, Sassy heads inside herself. While Shadow finds Tucker and gets him out of the house moments before the front door is hit with a fireball from a backdraft, causing the porch to collapse, Sassy soon emerges herself with Tiger in her mouth, unharmed as well, though Sassy doesn't feel the same way...
- “Somebody get this little pipsqueak out of my mouth! ... I'm singed and I smell like an ashtray...”
- ―Sassy grumbling about surviving a fire to save one of her own kind
After Tucker thanks Sassy and Shadow for saving both Tiger's life and his own, the two of them regroup with Riley and his gang, who had witnessed the entire thing, and impressed by Shadow and Sassy's heroism, Riley agrees to have Shadow and Sassy stay with his gang until they can find Chance and then head home.
However, when they arrive back at Riley's hideout, they find Chance and Delilah already there, and Sassy is livid when she sees Chance, as is Shadow. However, when Riley tries to convince Delilah that she and Chance can't be together because Delilah is a stray and Chance is a house pet, Chance and Delilah leave to blow off some steam. Later, Sassy is startled awake when she finds she was sleeping next to Stokey, startling him awake by her outcry, but soon enough, she learns that the "Blood Red Van" has arrived in the dockyard, and can only watch when Chance, who was distracted by some garbage, is captured by Jack and Ralph. As Shadow, Riley, Sassy, and the rest of Riley's gang attempt to head the van off, when they do, Riley says they need a plan to rescue Chance and stop Jack and Ralph from escaping, at which point, Sassy chirps up, saying "the cat has a plan".
That plan being having the dogs form a roadblock to prevent the van from leaving. When Jack sees them, he tells Ralph to run them over, but Ralph is hesitant to do so, knowing the big bucks that could be earned from capturing all of them. Forced to take matters into his own hands, Jack tries to force the van into advancing towards the roadblock as Ralph tries to stop him, soon forcing Riley, Shadow, and most of the gang to scatter, but Delilah holds her ground, refusing to budge because of her concern for Chance. When the van nearly runs her over, she fakes being injured, leading to an infuriated Bando attacking Jack and Ralph and chasing them out of the cab. With them gone, Shadow signals Sassy to make her move, and she heads around back to free Chance and the other dogs.
- “Before I set you free...Say it!"
"No, not in front of the guys! Alright, cats rule."
"Alright! Cats rule!”
- ―Sassy forces Chance to once again admit that cats rule and dogs drool
After getting Chance to admit that once again cats rule and dogs drool and get some payback for his earlier rephrasing of her motto, she lets Chance and the other dogs go and Bando sees to the end of the "Blood Red Van" while the captured dogs chase off Jack and Ralph. Afterward, once Bando tells off Chance for putting all of them in danger and Delilah realizes she and Chance can't be together, Chance leaves in heartbreak, and when Shadow tries to stop him, Chance angrily tells him off for his and everyone else's actions, clearly angry with him and everyone else for trying to keep him and Deliah apart, saying he doesn't need Shadow, Sassy, or anyone else and continued running away. By nightfall, Riley said he had his gang look for Chance, but found no trace of him, so Riley tells Shadow and Sassy that if they want to head for the Golden Gate, they have to do it now so they will be there by morning. With little choice at this point, Shadow and Sassy agree, and Riley takes them to the Golden Gate. Once there, Riley parts ways with Shadow and Sassy, but on better terms as he'll take what they said about humans to heart. As Sassy and Shadow head for the bridge, they stop in a construction yard as Shadow doesn't feel right about crossing the bridge without Chance. Sassy apparently is happy to go home without Chance, but Shadow makes it clear they can't cross the bridge without him. Realizing he's right, Sassy curses at how close they got before Shadow declares they are heading back to find Chance, only to be ambushed by Ashcan and Pete, who want to finish what they started earlier. Sassy tries to flee, but Ashcan sends Pete after her, and Sassy gets a good lick in on Pete when she lashes out and scratches him with a paw with a yowl while Shadow squares off with Ashcan again. Thankfully, Chance returns and deals with Ashcan and Pete, using the same trick he used on Sassy earlier to incapacitate them. Upon reuniting with Shadow and Sassy, Chance explains why he came back, and while Sassy is initially thankful, she quickly snaps and angrily asks what took him so long to do so, but becomes ecstatic again afterward as she asks her boys if they are ready to go home. Shadow fully agrees with her, but Chance is not so sure, still worried Jamie is still furious with him.
After crossing the Golden Gate Bridge, the pets only have to cross one more road and hill before they will be back home, but while Sassy and Shadow cross no problem, Chance stops halfway across to look back at San Francisco, still heartbroken over leaving Delilah. When Sassy and Shadow notice, Sassy tries to call Chance by mentioning food, but even that doesn't work, and Shadow suggests giving Chance time and he'll come around.
- “Sassy, do you hear a truck?!"
"No, Shadow! I see a truck!"
"CHANCE! LOOK OUT!"
- ―Shadow and Sassy realize Chance is in grave danger and doesn't know it
However, Shadow soon hears an ominous rumbling coming around the bend and turns to Sassy, asking if she hears a semi-truck. Sassy says she doesn't hear a truck, as she instead sees it when said truck rounds the bend, heading straight for Chance, who is oblivious to the danger. Shadow barks to Chance to look out as Sassy also cries out for Chance to take notice, but it is not until the trucker sees Chance, slams on his brakes, and blares his horn does Chance notice, possibly too late. Unknown to the pets, the Seavers were right behind the truck, on their way home, as Hope believed that the pets made it, and barely avoid crashing into the back of the truck themselves when they saw it hit the brakes. After Bob and Laura make sure that the kids are alright, they prepare to get out to speak to the truck driver, when Peter and Hope spot Shadow and Sassy coming down the hill to check on Chance. As soon as they point them out to their parents, Bob and Laura are shocked that the pets actually did make it home as they thought. Sassy reunites with Hope, but as Jamie calls out to Chance, Sassy asks Shadow as to if Chance made it or not. Thankfully, he did see the truck in time and cowered down to avoid being hurt, and happily reunites with an apologetic Jamie.
Sometime later, with everyone back home, Sassy notices Chance is still down in the dumps lately and admits that she misses the "Old Chance." She also attempts to eat some food, although Hope tells her not to stand on the table, causing Sassy to casually state that the "Honeymoon" was over. However, when Delilah shows up, having followed her heart and it telling her to be with Chance, with Riley having allowed her to leave his gang to be with Chance when Bob agrees to allow Delilah to stay and join the family, Chance leaps for joy, and Sassy can tell that the Chance she and Shadow know and love is back before Hope, Jamie, and Peter welcome Delilah into the family.