'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.
On the first day of August, Phineas and Ferb create a serum that will give anyone who drinks it near-unlimited power and great knowledge. When his mother tells Phineas that Isabella has an undiagnosed disease, he decides to give it to her. Meanwhile, Perry is trying to infliltrate 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, the amount of knowledge drives her insane. She destroys the hospital with a giant explosion caused 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. Phineas and Ferb, meanwhile, have gathered up the survivors inside their house, hoping for a savior.
The "Too Young" Line
Dr. Hirano: Aren't you a little young to be making miracle cures?
Phineas: Yes, actually.
To Phineas: Nice going.
Phineas to Isabella in the hospital (just like in I Scream, You Scream)
- Phineas says whatcha doin' ("I Scream, You Scream","That Sinking Feeling","At the Car Wash")
Spiderman: The serum is similar to the Goblin Formula used by the Green Goblin to give him his powers, which is slowly driving him insane.