Candace Flynn is a teenage girl who lives in the Tri-State Area, and the sister of Phineas Flynn and Ferb Fletcher. She is the leader of The Resistance, a group of civilians who seek to overrule Heinz Doofenshmirtz. ("Phineas and Ferb The Movie: Across the 2nd Dimension")
Candace Flynn, from the second dimension, was presumably born on the 11th of July. Candace's childhood was the same as her 1st Dimension counterpart: her mother Linda met and fell in love with Lawrence Fletcher. The two later got married and Candace and her brother Phineas gained Ferb as a stepbrother. At least five years prior to Across the 2nd Dimension, Phineas and Ferb bought Perry, who apparently was a secret agent (it is never brought up). Somehow, Perry was defeated and captured by Dr. Doofenshmirtz; he was then turned into the Platyborg, a cyborg platypus that commanded an army of Norm bots to invade The Tri-State Area and conquer it. As a result, she, like everyone else in Danville and the rest of The Tri-State Area, was forced to wear Dooferalls and play Doofopoly. At some point, Candace decided that she had enough of Doofenshmirtz's tyrannical reign and founded The Resistance, an organization bent on putting an end to Doofenshmirt'z rule. She forbade Phineas or Ferb from joining it for their own safety. Doofenshmirtz and the Resistance have since been at odds with one another ever since, paralleling the life-long feud between the Perry and Dr. Doofenshmirtz of the 1st Dimension.
Candace meets Phineas-1 and Ferb-1 at the Flynn-Fletcher household, but aside from a simple question about their presence, to which she receives an irrelevant reply from Phineas-1, she doesn't take much note of it.
Her secret is later exposed to her brothers and their 1st Dimension counterparts when they discover the Resistance headquarters while being escorted by former Resistance member Buford. Initially, she refuses to help them in their quest to rescue Perry-1, saying that it's "their fight", but she changes her mind and takes the two, along with a recently-arrived Candace-1, Buford, and Isabella, aboard mine carts used by the Resistance as means of transportation to Doofenshmirtz Evil Incorporated. During the trip, she interacts with Candace-1, telling her what it's like to be a teenage girl in the 2nd Dimension before she finds out that Phineas and Ferb have secretly stowed away on the mine carts.
Upon arriving underneath DEI, Candace becomes suspicious of the mission currently being easy, and when Phineas-1, Ferb-1, Candace-1, Isabella, and Buford are nearly captured by Doofenshmirtz and Doofenshmirtz-1, who have set a trap for them, she takes them and flees in the mine cartsw along with her own brothers and Perry-1. However, a resultant chase by the Norm bots forces her to separate the mine cart containing the 1st Dimension characters from the rest to avoid letting Phineas and Ferb being captured. Phineas tries to protest, but she does it anyway.
However, she apparently changes her mind on helping the 1st Dimension characters, revealing her presence in the Goozim room and rescuing Phineas-1, Ferb-1, Perry-1, Candace-1, and Doofenshmirtz-1 from certain doom by tossing a remote version of Doofenshmirtz-1's Otherdimensionator to Phineas-1, who uses it to travel to the 1st Dimension along with the other four. Unfortunately, this comes at a price: she is captured by the Norm bots and sentenced to a cell.
Entertaining herself with a ball, Candace is suddenly saved by Jeremy, Phineas, and Ferb, the latter two having been recruited into the Resistance. At first, she disapproves of this, but upon realizing that the two can handle themselves, she changes her mind and gives them a hug.
She later congratulates the 1st Dimension characters for their efforts in ultimately defeating Doofenshmirtz, and she admits to Candace-1 that she spent her entire life trying to bust Doofenshmirtz that she doesn't know what to do with her life now. Candace-1 replies that she can now pursue other, more average things (such as becoming Jeremy's girlfriend), and the two share a hug before Candace departs into the 2nd Dimension with her brothers and colleagues ("Phineas and Ferb The Movie: Across the 2nd Dimension").