Isapocalypse Now

'Isapocalypse Now' is the third-to-last episode of the fourth season of Phineas and Ferb. It is also the first part of the epic 'Dark Isabella' saga.

Summary
On the first day of August, Phineas and Ferb create a serum that will give anyone who drinks it near-unlimited power. When his mother tells Phineas that Isabella has a muscular disease, he decides to give it to her. Meanwhile, Perry is trying to infiltrate Doofinshmertz's new hideout in Australia, where he discovers that the doctor has created a device that will transport all major cities to Mars. At the hospital, Isabella drinks the serum and is instantly cured. But she continues to gain power, and the thrill of it drives her insane. She destroys the hospital and begins to decimate the city. Candace has been at a nail salon all episode, but she runs out after Dark Isabella tears off the roof. In Doofinshmertz's hideout, he and Perry are battling, but Dark Isabella destroys them and the building. Phineas and Ferb, meanwhile, have gathered up the survivors inside their house, hoping for a savior.

'Aren't you a little young'
Doctor: Aren't you a little young to be making miracle cures? Phineas: Yes, actually.

Ferb's lines
To Phineas: Nice going.