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...

Kat and Doofenshmirtz were walking one day when Kat sat, and wouldn't get back up. Was she just being a stubborn brat, or was something wrong? Then, he noticed how pale her fur was. Was his best pal getting old? Kat had very bad arthritis in her hips. They often collapsed on her. She was deaf, and was almost blind. One day, she collapsed on her old, shot, creaky, hips, and wouldn't get back up. Luckily, the vet saw. After a few long moments, the vet came out and said they would have to go into serious surgery. Doofenshmirtz went back with the vet, who was about to give her a shot. "There's...