As the two boys are sitting underneath the tree in the backyard, Phineas tells Ferb that one of the best times they ever had was when they built the rollercoaster. He proposes that they do it again, only this time as a musical— breaking into spontaneous music and choreography without a discernible music source. Ferb wonders if they would have any assurance that everyone else would break into song and dance along with them. Phineas thinks they probably will, so Ferb accepts. Phineas then starts singing the first musical number Hey Ferb (I Know What We're Gonna Do Today).

Linda tells Phineas...