When Worlds Collide 《Ferbnessa》

When sixteen year old Vanessa Doofenshmirtz takes a job babysitting her friend Candace's younger brothers Phineas and Ferb, she has no idea of the adventure that she's getting into. Especially when her father, Doctor Heinz Doofenshmirtz, invents a device called the De-Age-Inator, which is supposed to turn his brother Roger into a child, thereby making him unable to be Danville's mayor and allowing him to take over. When the Inator hits Vanessa after Doofenshmirtz accidentally fires it while explaining his evil scheme to his nemesis, Perry the Platypus, she finds her feelings for Ferb beginning to change. While all of this is occurring, Doofenshmirtz begins working on an Inator to reverse the effects of the De-Age-Inator.

I don't own Phineas and Ferb, and all rights belong to Disney and the creators of the show and characters, Dan Povenmire and Jeff "Swampy" Marsh.