Phineas and Ferb create a monster truck for Candace to practice driving while Dr. Doofenshmirtz tries to find investors for his line of monster trucks that are actual monsters, with a trump card to take down the competition.

Phineas and Ferb build a shrinking submarine so they can go inside Isabella's dog and retrieve a sash the pet ate, but they accidentally end up in Candace's stomach instead. Meanwhile, Perry must stop Doofenshmirtz from destroying people who can't make up their mind.