'Reign of Isabella' is the second-to-last episode of the fourth season of Phineas and Ferb. It is the second part of the epic 'Dark Isabella' saga, following Isapocalypse Now and preceding Last of the Fletchers.
Dark Isabella has become a threat to world security. A special ops team infiltrates Danville in a search for Phineas and Ferb. After they are found, they employ the stepbrothers to create a weapon that will neutralize Dark Isabella. They successfully create it: a huge laser in space that will drain all the evil from Dark Isabella. Without the government's knowledge, Phineas and Ferb fly away on a rockat-powered glider to Dark Isabella's current location, South America.
In South America, Dark Isabella is destroying the rainforest with her evil powers to make way for a city built in her image. When she sees Phineas, she tries to convince him into becoming her consort. Phineas refuses, so Dark Isabella threatens to destroy a group of government agents that tracked the boys, along with Ferb, Candace, their parents, Perry, Baljeet, and Buford. Phineas still refuses. Isabella shoots a laser. Phineas, not willing to let innocents die, intercepted the laser, dying to save many. Ferb, inspired by his stepbrother's willing to die, vows to continue what his brother started: defeat Dark Isabella and avenge Phineas.
The "Too Young" Line
Black Ops Agent: "Aren't you a little young to have created a crazy supervillainess?"
Phineas: "NOOOOOOOO!" (Phineas intercepts laser and dies, Isabella teleports away.)
Ferb: "I swear your grave, Phineas, I will finish what you started and defeat Dark Isabella. I will avenge you!"