It's a new summer for stepbrothers Phineas and Ferb, and their friends. The next morning, however on Friday, Phineas and Candace, who had a falling out the previous day, wake up to realize they've mysteriously switched bodies! Until they figure how to switch back, they are forced to take over each other's roles, and come to realize what they do and how much it means to them.

After the supposed death of their father, Mandy and Robyn Starling are left under the care of their wicked Aunt Pristine Figg. Having had enough of this abuse, the sisters decide to run away, where they befriend everyone's favorite cat and mouse, Tom and Jerry. The duo are reluctantly forced to put aside their rivalry in order to save both girls from their evil aunt.