The boys compete with a pair of visiting brothers who also like to build ambitious projects to see who can construct the best backyard fort. Meanwhile at a family reunion, Dr. Doofenshmirtz tries to win a game of kickball with a robotic kicking leg.
Phineas and Ferb build a passenger plane out of papier-mache to rival the famous Spruce Goose. Meanwhile, Candace and Stacy visit a pool party at Jeremy's house, and Doofenshmirtz builds an evaporation machine out of recycled materials in retaliation for his mother not letting him swim in public pools.