Maisie is first seen in a cage at Rob's lab with her three newborn kittens who are currently blind. After Ratso makes a rude comment about Maisie and her kittens, Maisie becomes offended and tells Ratso to watch his mouth, as she dislikes foul language. When Ratso approaches Maisie, she covers her kittens with her tail and warns him to stay away. Ratso assures Maisie that he isn't going to hurt her, but the latter explains that she is worried about her three babies who have not opened their eyes yet, and that she doesn't want Ratso to be the first thing that they see. Ratso retorts, only to be whisked by Maisie's tail.
Maisie is later present during the "Remember That Day" sequence and recalls the time when Rob (the Master) first found her stranded in a rainstorm and took her in. Presumably after this, she had given birth to three baby kittens. Sometime later, Maisie and the other animals learn that Mack plans to ship them to Tartarus Laboratories to be used as test subjects. Maisie is scared, but says she will go wherever she is taken and deal with it, but doesn't want her babies threatened.
Ratso later leads Toaster and company to a recent disturbance that was coming from the basement. Maisie calls out to Ratso and wishes him good luck, to which he smiles back and salutes Maisie, indicating that he is now on good terms with her. Mack shows up shortly after and prepares to ship the animals. He attempts to steal one of Maisie's kittens, but she manages to scratch Mack on the wrist. Kirby then chases Mack around the lab, only for Mack to lock Kirby in a closet.
When Maisie and the other animals are eventually shipped by Mack, Toaster and the gang chase down the truck. When they manage to catch up with the truck, Ratso asks if everyone is okay and suddenly notices Maisie trapped under a wooden crate. Ratso and Toaster help pull an unconscious Maisie out from under the wreckage. Ratso sheds a tear, believing Maisie is dead, but she is revealed to be alive after her three kittens lick her face lovingly. Maisie congratulates Ratso for being a hero, and Ratso smiles good-naturedly while blushing.
Back at the Master's lab, Maisie calls a sleeping Ratso to her cage and tells him that her three babies are just starting to open their eyes, and that she wants Ratso to be the first thing that they see. When the baby kittens slowly open their eyes and notice Ratso, they pile on top of him and lick his face, much to Ratso's pleasure. It is later revealed by Chris that Maisie and her kittens have been adopted and are going to be the Sigma Chi mascots.
Maisie is last seen residing in her new owner's home with her three kittens during the "Hang in There, Kid" sequence and waving to Rob and Chris as they drive off to start their new life together. She also appears briefly on Wittgenstein's computer screen with all the other protagonists of the film.