When Phineas and Ferb's group of friends (Isabella; Bufford Baljeet and the explorers' troop) go on holidays, Phineas and Ferb feel alone so they invent a way of having new friends: to travel to other worlds, where they finish in Springfield and ask Homer if he has any children, the one who answers them with another question; Where is Perry?
In the lair (Perry accedes across the Kwik-E-Mart); Monogramm says to Perry that his nemesis Doofenshmirtz is planning something in the power plant.
While in Evergreen Terrace; Ferb falls in love with Lisa and Phineas along with Bart prepare a date for them with a musical number. In the Plant Heinz and Mr. Burns are revealed to execute a plan with the Power-Plant Inator with which they will produce nuclear power free.
During a battle, the invention is destroyed and the nuclear energy creates hearts that float into the heavens; Lisa thinks that Ferb made it and kisses him. At last Phineas says that there would be a second part.
The "Too Young" Line
Homer: Aren't you a little young to have a Cross-over?
Phineas: Yes, yes we are.
Phineas: What happened?
Ferb: I fell in love.
"It's Difficult to be a Bookworm" Sung by Ferb.