In Manhattan, Max is enjoying his life as his owner Katie's favorite pet in the comfortable apartment building where they live together. He loves her so much that he sits by the door, just waiting for her to come home. But one day, his life is turned upside down when his human brings home a sloppy mongrel named Duke.
Duke turns out to be less than careful about Katie's decor, which gives Max an idea. He decides to break some things and blame it all on the new guy. But when Max and Duke get out of the apartment and are captured by Animal Control, the other pets in the neighborhood hatch a plan to rescue them.