'Last of the Fletchers' is the last episode of the fourth season of Phineas and Ferb. It the finale of the 'Dark Isabella saga.'
Following the events of the previous episode, Dark Isabella has completely taken over the planet Earth. Most of the monuments have been rearranged to feature her image, and no one can even think about revolting. However, Ferb is travelling across the planet, and finally arrives in the ruins of Danville. He meets Baljeet, whom he and Phineas had entrusted with the formula that created Dark Isabella. Baljeet gives Ferb the formula to alter it so the drinker won't gain power. He decides to travel back in time so he can swap the old formula with the powerless formula. He heads to an abandoned warehouse which is rumored to be where Dark Isabella stores a time portal. Ferb enters, but dark Isabella is there. They fight, but Dark Isabella is defeated. She said she was fighting the insanity and that she'd never mean to hurt Ferb. She uses her telekinesis to lift a sharp metal rod and plans to stab Ferb from behind. But Ferb dodges the rod, and Dark Isabella stabs herself. Ferb hops into the portal and tackles past Ferb. He explains to his past self that the formula would drive Isabella insane and cause Phineas to die and that she would rule the world. So, he switches the formula with the powerless one and hops back in the closing portal and returns to a bright and happy present. After that, he ceases to exist, but with satisfaction that he saved the world from a much worse fate.
The 'Too Young' Line
Baljeet: Aren't you a little young to be saving humanity?
Baljeet: "Why do you need the formula?"
Ferb: "Because I want to alter it and give it to my past self so this future won't occure."
Baljeet: "Okay, here you go."
Isabella: "I was trying to fight the insanity. I'd never try to hurt you or Phineas. I swear. (Rod positions itself behind Ferb.) Goodbye, Fletcher."
The rod zooms at Ferb, but he dodges it and it stabs Dark Isabella.)
Ferb: "Missed me! (Hops through the portal and arrives in the Danville General Hospital three days earlier and tackles past Ferb.)
Past Ferb: "What are you- How did- are you from the future?!?!"
Ferb: "Yep. So listen: You have to swap the formula with this one. If you don't, Isabella will go insane and kill Phineas and she'll rule the world!"
Past Ferb: "Okay. THAT seems like a pretty good reason." (Ferb swaps the formula and enters the slowly shrinking portal, which closes. Past Phineas walks up to a visibly shaken Past Ferb.)
Past Phineas: "C'mon, let's give the formula to Isabella."
Past Ferb: "O-okay."
(The screen changes to Ferb in the present day, and a wave sweeps over Danville, reverting it back to its normal state. The wave passes over Ferb, and he begins to disappear.)
Ferb: "My work here is done." (Ferb disappears fully.)