Invasion of the Ferb Snatchers/Ain't No Kiddie Ride

Phineas and Ferb travel to the future to find a tool that hasn't been invented yet, and run into future Candace. Seeing an opportunity to finally bust her brothers, she follows them back through time to catch them building their rollercoaster, and exposes their antics to her mom. In the process, she accidentally foils Agent P's defeat of Dr. Doofenshmirtz, starting a chain reaction that turns the future into a dystopian society.