Ratso is first seen in Rob's lab. Here, he is shown to be very grumpy and angry, as if mad about being kept as a pet, and doesn't believe the Master (Rob) is wonderful. This is possibly because of his past experiences with humans who screamed at him, threw things at him, and called the exterminator on him. He is rude to pretty much everyone, but as the film goes on, Ratso's heart begins to warm up to the others.
When Ratso senses a disturbance in the basement and informs Toaster and the gang, they investigate the cellar and meet Wittgenstein, an old supercomputer who had been abandoned and left in cold storage. Ratso and the others learn that he is plagued by viruses and living on one rare cathode tube called the WFC 11-12-55. Ratso then volunteers to go to the college's storage building to find the hard-to-find WFC 11-12-55 tube to revive Wittgenstein, and Radio tags along to make sure Ratso finds the correct tube. When they come back with the last apparent tube, Radio and Ratso have a brief fight before Radio accidentally breaks the tube. Ratso scolds Radio, who then sacrifices his own tube to give old Wittgenstein the spark he needs to help them save the animals from being shipped to Tartarus Laboratories.
After the gang manages to reach the truck, Ratso pulls Maisie out from under some boxes and mourns over her, believing she is dead. However, she then wakes up and calls Ratso a hero, to which he responds by blushing good-naturedly while holding his tail. The next morning, Maisie calls a sleeping Ratso to her cage and allows him to be the first thing for her babies to see. Ratso is happy to play with the kittens after they tackle him and lick him. Later, when Ratso hears that the other animals have homes except him, he sadly sulks. However, he is overjoyed when he hears Chris say that he's coming with them, stating that she "thinks he's kinda cute." Ratso is also happy when the couple revive Radio with a new replacement cathode tube. In the end, Ratso leaves for his new home with Toaster and his friends, while waving goodbye to the other animals and singing "Hang in There, Kid".
Ratso is first seen in Rob and Chris's new home. Here, he is shown to be very kind and helpful to Toaster and the other appliances. When the parents' infant son Robbie gets abducted and taken to Mars, Ratso stays behind and stands watch by keeping the Baby Monitor from going off, while Toaster and his friends go on a mission to rescue Robbie, who is referred to as the "Little Master" by the appliances. At the end of the movie, Ratso is seen playing tick-tack-toe with the Microwave, where the former gets beaten by the Microwave.
- Ratso bears some resemblance to The Rat from Lady and the Tramp, but unlike Ratso, he's an antagonist.
- He is the second Disney rat character to be a protagonist, the first being Snitch from Bonkers, the third being Remy from Ratatouille.