While Phineas and Ferb convert the home into a funhouse, Candace takes Perry to a charity pet washing so she can spend time with Jeremy. Though a strange sequence of events, another platypus that was on display at the local zoo takes Agent P's place for his one hundredth battle against Dr. Doofenshmirtz, who has created a Least Likely-inator to make anyone affected by it do the opposite of what they would usually do.