Katie the Catsitter is a novel that is very concerned about animal rights and the relationships that human beings have with animals. This is evident in the fact most of the action revolves around Katie learning to care for 217 highly intelligent cats, and through Madeline, who is eventually revealed to be the Mousetress. By day, Ms. Lang campaigns for animal rights through more traditional, accepted means, such as the protest she speaks at, and by running a chain of vegan restaurants. By night, she dons the Mousetress costume and commits much more radical acts, such as starting fires, blowing up buildings, and breaking and entering. The novel uses animal rights as a means to explore social activism more broadly, especially its intersection with the law, ethics, and morality.

The Mousetress is portrayed as a benevolent vigilante. Her three targets are not innocent victims, but are guilty of animal abuse and cruelty: New You Cosmetics was engaging in animal testing; Pure Organics owner Lydia Staples was illegally hunting endangered animals; and celebrity Hunter Q. Prescott ran an illegal gambling ring that specialized in dogfighting. At the protest over mistreatment of carriage horses, Ms. Lang’s speech extends the novel’s definition of animal cruelty to include taking overworking animals and keeping them in conditions that result in suffering and a poor quality of life.