Pibby and BF leap through a rift into a new realm, which happens to be the world of "Phineas and Ferb." They must dash into Dr. Doofenshmirtz's building to evade the encroaching corruption, where they encounter a small teal platypus. This platypus turns out to be Perry, also known as Agent P. Despite living as the pet of Candace, Phineas, and Ferb Flynn, Perry leads a secret life as a skilled agent, fully equipped to face the corruption. He even joins in a song with BF. However, lurking behind Perry is his nemesis, Dr. Doofenshmirtz, who has also fallen victim to the Pibby Corruption, with black sludge marring various parts of his body. A menacing ray-like device protrudes from his back, enhancing his threatening appearance. Now, Perry must join forces with BF and Pibby to confront his tainted adversary.