In Across the 2nd Dimension, Phineas (Vincent Martella) and Ferb (Thomas Sangster) are celebrating the fifth anniversary in which they got their pet platypus, Perry (Dee Bradley Baker). They create a figure of Perry and it crashes into Heinz Doofenshmirtz's (Dan Povenmire) laboratory.
Phineas, Ferb help Dr. Doofenshmirtz rebuild his "Otherdimensionator" where Perry shows up and then they find themselves in an alternate universe where a second, truly evil Dr. Doof rules over his Tri-state Area with an army of iron-fisted robots.
To save his friends from certain doom, Perry makes the ultimate sacrifice by revealing his secret identity as Agent P. Phineas and Ferb escape, meet their 2nd dimension selves, during an escape from the 2nd Dimension Doofenshmirtz.
Falling down Doofenshmirtz Evil Inc., Perry inserts his legs in a skull's eyes and saves Phineas and Ferb, but when they reach the building below, Perry's legs get stuck in the skull's eyes from which he gets captured, the boys escape and begin their own mission to rescue Perry. As they uncover his plot to travel back to their dimension to take over their Tri-state Area, the gang, as well as their alter-egos, must band together to stop him. So begins an epic battle as Phineas And Ferb lead a resistance to try to save their home from the clutches of Evil Dr. Doofenshirtz and his sidekick Platyborg.
Source: Disney Channel