Ferb Fletcher is the brother of Phineas Flynn and Candace Flynn. Originally a laconic stay out of trouble boy, Ferb joined The Resistance and plotted to override the rule of Heinz Doofenshmirtz. ("Phineas and Ferb The Movie: Across the 2nd Dimension")
Ferb's early years were presumably the same as his 1st Dimension counterpart: his father Lawrence met Linda Flynn and fell in love with her, eventually marrying her, and he gained Linda's children Phineas and Candace as step-siblings. At least five years ago prior to Across the 2nd Dimension, he and Phineas bought Perry, who apparently was a secret agent (it is never brought up). Somehow, Perry was defeated and captured by Dr. Doofenshmirtz without Ferb's knowledge, resulting in his transformation into the Platyborg, which commanded an army of Norm bots to the invade The Tri-State Area and conquer it. Now forced to live under Doofenshmirtz's tyrannical reign, Ferb spent the recent years playing Doofopoly with Phineas while wearing his Dooferalls. At some point, Candace founded The Resistance, an organization bent on overthrowing Doofenshmirtz, but forbade Ferb or Phineas from joining it out of concern for their safety
Ferb and Phineas meet their 1st Dimension selves while playing Doofopoly, and he and Ferb-1 exchange handkerchiefs while watching Phineas hug Perry-1 under the impression that he is their Perry. After Perry-1 departs, the four go to Isabella's house; during the walk, they are saved by Buford from a patrolling Norm bot. The five then arrive at the Resistance headquarters via chute, and he and Phineas, along with Phineas-1 and Ferb-1, find out that Candace is the leader of the Resistance.
Candace agrees to aid Phineas-1, Ferb-1, and a recently-arrived Candace-1 in rescuing Perry-1, who was captured by Doofenshmirtz. However, he and Phineas are told to stay behind; despite this, they stow away in mine carts used by the Resistance. When Perry is rescued and an army of Norm bots pursues them as a result, Candace is forced to separate their mine cart from that of the 1st Dimension characters' to save her brothers from certain capture.
After managing to become a Resistance member along with Phineas, Ferb aids Jeremy in his mission to rescue a captured Candace. She initially disapproves of her brothers' decisions, but after realizing that they can handle themselves in the outside world, she changes her opinions and shares a hug with the two of them.
Ferb is later amongst the Resistance members that travel to the 1st Dimension to congratulate the residents in defeating Doofenshmirtz. He and Phineas share a loving reunion with Perry, whose evil was "fried out of him" after a battle with Perry-1 ("Phineas and Ferb The Movie: Across the 2nd Dimension").