Peter was first seen outside their house attending the wedding of his mom and stepdad. He hands a frog to his brother Jamie, who puts it in the hand of their sister Hope, who freaks out and drops the frog on the ground, causing Jamie to burst out laughing. Peter then hears a crash coming from inside the house, and Shadow, his pet dog, growls at the noise. It turns out it was only a lampshade being knocked down by Chance, Jamie's pet dog, who has been annoyed by Sassy, Hope's pet cat, and Peter tries to get Shadow to relax and calm down. Peter and Shadow are playing basketball and witness Sassy being catapulted from the see-saw into a sandbox by Chance and he and Jamie burst out laughing at Sassy's misfortune. This causes Hope to call Chance a stupid dog and report it to their mother.
After the wedding, Peter and his family go to the farm owned by their friend Kate, where they leave Shadow, Chance, and Sassy to be looked after by Kate's ranch hand, Frank. Despite being fearful that the pets, especially Shadow, would view themselves being left behind as being abandoned, Peter sadly leaves with his family. At their home in the city, Peter gets a call from Kate who then tells him that Shadow wants to talk to him and so he did. He is again seen at the school drawing Shadow during the class.
A few days later, Peter and his family learned that Shadow, Chance, and Sassy are no longer on the farm from Kate and her ranch hand Frank, due to them having left the farm to return back home. Jamie and Hope are weeping at this, while Peter was upset and their father refuses to go out to look for them and have to be patient and wait until any word from the forest rangers. Peter thinks that he was right about the animals feeling like they were abandoned and angrily scolds his father for leaving them at the farm and returns to his room. Afterward, Peter goes to the police station to try to get further assistance. However, he is refused by the sergeant, who responds that chasing after lost pets is not a top concern for the SFPD. After Peter retorts that the missing pets are important to him and his family, he is advised to put up reward posters for the animals, with the sergeant wishing him good luck, showing no ill will to Peter for trying to make him think the pets were important enough for the police to track down.
He is later called by his step-father who had received a call from the forest rangers that they finally found Shadow, Sassy, and Chance, who helped a lost girl named Molly who also been looking by her parents. He and his father report it to Laura, Hope, and picking Jamie up, who is in the middle of practice of a play, they are soon on their way to the shelter where they are currently held. However, by the time they arrived, due to Chance mistaking the shelter for a pound, the animals had broken loose and dug their way out. Peter calls out Shadow in the darkness, but no barking is heard. The shelter staff apologizes to the Seavers for failing to stop Shadow, Chance, and Sassy from escaping, and the Seavers sadly return home.
By dusk, while playing basketball Peter begins to accept Bob as a second father figure, and Jamie hears Chance barking in the distance and calls out for him. Sassy then arrives shortly after and runs up to Hope. Chance and Sassy finally arrive and reunite with their owners, except for Shadow having fallen into a muddy pit at the train yard and Peter laments that Shadow is too old to make it back home. But Shadow appears in the distance limping (having injured his leg from the fall and made out of the muddy pit) having caught up to Chance and Sassy and is happily reunited with Peter and they play together.
Years later since the last time the children were seen, Peter and his siblings have grown up and lives in San Francisco and Bob has adopted him and Hope and Jamie. Peter and his family are getting ready for their family trip to Canada.
At the airport, Peter and his family put Shadow, Sassy, and Chance in their cages before they are ready to board the plane. In the plane, Peter then asks his father if Shadow, Chance, and Sassy are going to be okay, and he replies that they are safe and sound below their feet.
However, when Peter and his family arrive at Canada, they receive word that Shadow, Sassy, and Chance had somehow gotten loose due to Chance escaping from his cage, mistakenly again thinking that he and the others will be take to the pound. They later called the animal authorities at San Francisco to look for Shadow, Chance, and Sassy.
Peter and his family returned home and hoping that their pets would be home. On the way, a container truck which was in front of their car suddenly skids to a stop, and Bob manages to avoid it by pressing on the brake, making their car to skid to a stop as well. Peter then asks what happened, and Bob replies it could be an accident. Peter then sees Shadow and Sassy climbing down the hill nearby, and he and his sister reunite with their pets. Bob talks to the truck driver who apologizes and asks if anyone is okay and he replies that they are okay and what happened. The truck driver says that there was a dog in the middle of the road. They find out it was Chance, who was in the middle of the road safe and sound, and he is reunited with Jamie.
At home, Peter and his family meet and adopt a new dog named Delilah, Chance's love interest whom Shadow, Chance, and Sassy met in San Francisco while on their way back home.