The show follows the adventures of stepbrothers Phineas Flynn (Vincent Martella) and Ferb Fletcher (Thomas Sangster), who live in the fictional city of Danville, in a (never specified) tri-state area, as they seek ways to occupy their time during their summer vacation. Often these adventures involve elaborate, life-sized and ostensibly dangerous construction projects. Phineas's older sister, Candace Flynn (Ashley Tisdale), has two obsessions: "busting" Phineas and Ferb's schemes and ideas, and winning the attention of a boy named Jeremy (Mitchel Musso). Meanwhile, the boys' pet platypus, Perry, acts as a secret agent for an all-animal government organization called the O.W.C.A. ("Organization Without a Cool Acronym"), fighting Dr. Heinz Doofenshmirtz.
Much of the series' humor relies on running gags used in almost every episode, with slight variation. Most episodes follow a pattern: