For completionists: Watch everything. The show has no true “bad” episodes. Phineas and Ferb is one of the most tightly written, musically gifted, and emotionally intelligent animated shows ever made. The 104 days may be over, but the rewatchability is infinite.
This season leans into serialized elements. Doof gets a tragic backstory almost every episode, and the show acknowledges its own aging.
This season establishes the ironclad formula: Phineas & Ferb build something incredible, Candace tries to "bust" them, Perry fights Dr. Doofenshmirtz, and Mom never sees the evidence.