As the two boys are sitting underneath the árvore 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 música and choreography without a discernible música 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 cantar the first musical number oi Ferb (I Know What We're Gonna Do Today).

Linda tells Phineas and Ferb that she's going to go pick up a few things. Before she leaves, Candace asks if she is in charge. Linda says nobody needs to be in charge, but Candace argues that she should be in charge in case of an emegrency. Specifically, if a herd of moose were to burst into the house and attack everyone. Linda says yes, if that happens then Candace can be in charge. Candace tells Phineas and Ferb, who are working on rollercoaster blueprints, that she is in charge “conditionally”. Phineas asks if the condition had anything to do with a moose. His correct guess frustrates Candace so she goes into the house. Just then Stacy calls Candace. Stacy and Jenny are finishing getting dressed in 60’s do-wop outfits. Candace and Stacy are discussing the varying degrees of authority, when Stacy asks what Phineas and Ferb are doing (commenting that she can see it from her house). Candace runs outside to find out. In the backyard, Phineas points out that Stacy's house is visible from the topo, início of the rollercoaster they are constructing. Candace demands to know what they are building. When Phineas asks if Candace likes it, Candace declares that once she tells mom about this, they are going down. Stacy and Jenny appear por her side in their completed 60’s do-wop costumes and they sing the segundo song of the musical, You're Goin' Down.

At the end of the song, Phineas comments that there was no discernible música fonte in the song as Candace, Stacy, and Jenny walk away down the driveway. Isabella passes them and says hi, asking how the number went. Candace says it was pretty good, but she is still angry. Isabella comments that it’s her turn now, and she enters the backyard por cantar Whatcha Doin' (song).

Isabella asks Phineas what he's doin', and Phineas tells her they are building a rollercoaster again. Isabella greets Ferb and comments on how he’s still a man of few words, but Phineas tells her that earlier Ferb had two lines. Phineas wonders where Perry is as said platypus enters his lair through a secret door on the side of the house. Major Monogram tells Agent P that Doofensmirtz has bought up eighty percent of the country’s tin foil. Again. After checking the accuracy of the denunciar with Carl, he tells Agent P to go do the same thing he did last time. After Perry leaves, Major Monogram comments on how the life of the field agent is so exciting, unlike his own current situation. Major Monogram tries to sing his own ballad called “My Whole World is the Screen” when Carl turns off the lights on him because he mistakenly thought Major Monogram was done. Major Monogram tells Carl not to bother turning the lights back on, as the moment is gone.

As Perry is flying to Doofensmirtz Evil Incorporated in his platypus-styled hovercraft, he covers his face with his hat when he passes por Phineas and Ferb. Phineas tells Ferb where he wants the solid rocket boosters to fogo and when to release the snakes. Phineas leaves to get mais snakes as Ferb continues to weld and build the Rollercoaster. Meanwhile Candace has found her mom at the grocery store and tells her to come início right now. Linda asks sarcastically if there is a herd of moose in the living room. Candace says no and mentions how she thinks the plural of Moose is “Mooses” or “Meese”, but Linda is pretty sure that Moose is an already plural form of itself. When Linda asks why Candace even came over to the grocery store, she says that Phineas and Ferb have built a rollercoaster in the backyard, to which Linda replies “You’ve already used that one”. As Linda walks away, Candace sings the fifth song called Mom Look. At the end of the song, Linda says Phineas is a little young to be a rollercoaster engineer. Candace agrees – The song Aren't You A Little Young to Build a Rollercoaster immediately starts playing as the floor manager in the assembly line plant sings about youth and brilliance and how Phineas is a little young to be using the machinery.

At the end of the song, Doofensmirtz is playing the piano and cantar a slightly extended version of his own evil jingle, commenting on how he should really finish it one day. Perry burst through the window, with Doofensmirtz telling Agent P that he is unsurprised as he activates his robotic arms to capture Perry. Doofensmirtz then explains to Perry that he’s covered the entire Eastern Seaboard in Tin Foil again. He’s going to use his giant magnet and his magnetism-magnifinator to pull the east in a westerly direction, thereby reversing the rotation of the earth. Doofensmirtz explains that this is completely different than the first time because he’s using the suffix “inator”, so it will work. He says that although he knows that there should be some tragic back-story on why he wants to reverse the rotation of the earth, he says he won’t go there, and explains why por cantar the song Back in Gimmelshtump.

Candace continues insisting that Linda go and see what Phineas and Ferb are doing, claiming that it’s different this time because it’s a musical. She sees a poster advertising Phineas and Ferb's rollercoaster and goes to get Linda. But a group of boys get there first and decide to take the poster, thinking that even though they didn’t get in free last time, they should take the poster anyway just in case. However, Buford grabs the poster from them, stating that if anyone was going to ride that rollercoaster, it would be him and not some "lousy extras”. Candace returns with Linda to a blank post determined to prove she’s not crazy. When Linda says there’s no poster, Candace screams. Linda asks if yelling at some cheese would make Candace feel better, and Candace says it might.

Major Monogram briefly reappears hoping that Agent P is having good luck on his mission, and he tries to sing his “My Whole World is this Screen” before being stopped again when the scene cuts to a view of kids running into a huge tent in Phineas and Ferb’s backyard. Ferb closes the line and gets on stage and taps the mike for sound check He does a little jig before backing away as Phineas rises up from underneath the stage. Phineas gets the crowd excited about riding "the coolest coaster ever" while Ferb plays guitar, drums, and does disco lighting simultaneously. As the rollercoaster climbs the first hill, everyone seems apprehensive and legitimately scared as Isabella asks Phineas if he’s sure this is safe. Phineas and Ferb respond por cantar the eighth song Rollercoaster (song).

The coaster passes por Doofensmirtz Evil Incorporated just as Perry frees himself por using his tail to ricochet a screw off a flexible overhead power cord into Doofensmirtz’s foot. This causes him to accidentally lean on the release button, freeing Perry. A battle ensues, but Doofensmirtz distracts Perry long enough to activate his magnet- magnifinator. However, the tinfoil is ripped off the buildings (instead of pulling the east west) and forms a giant two ton ball of tinfoil that is moving at 200 miles per hora towards them. Doofensmirtz mentions how this seems like a serious déjà vu moment. Meanwhile, Candace is still yelling at some cheese when she hears kids on the rollercoaster screaming. After looking out the store doors and seeing the rollercoaster, she runs to go get Linda. As Doofenshmirtz and Perry are trying to dismantle Doofenshmirtz's invention, Perry spots a nearby helicopter. por attaching his grappling hook to the helicopter and the magnet, he saves himself while Doofensmirtz gets hit por the ball-- after again commenting on the serious case of déjà vu. He curses his déjà vu moments as the ball bounces and rolls off into the distance.

The magnet on the helicopter has attracted the magnifinator, which pulls the entire rollercoaster into mid-air just as Linda arrives. Candace hurries to get Linda início before the boys do. Perry cuts the magnet from the helicopter and lands on the rollercoaster por Isabella. With the Rollercoaster cut from the original tracks, it goes off course. The rollercoaster still follows the same path and trajectory as it did in the first episode, namely the construction site and the Jet Plane. They are fling por the Statue of Liberty to Mount Rushmore where they are then flung past a huge herd of moose with Phineas commenting that if they break into their house, Candace is in charge. A huge árvore stops them long enough to say hi to Jeremy at Mr. Slushy Burger, and then they are flung over to the Eiffel Tower in France where they bend the Eiffel Tower over long enough to be asked for a “crepe suzette”. The rollercoaster is then flung into o espaço where Phineas and Ferb say hi to Meap, and then they come crashing through the atmosphere towards earth.

Candace and Linda make it início just moments before Phineas and Ferb crash land in the tree, but por the time Linda turns around, Phineas and Ferb are sitting under the tree, much to Candace’s horror. As the kids leave, Isabella tells Phineas how great the ride was and asks what he’ll do tomorrow. Phineas replies “carpe diem” and explains that it’s Latin for “Seize the Day”. Ferb states that you could say that “Seize the Day” was English for “Carpe Diem”. Phineas replies, “Yes, Yes you could” and then the entire cast sings in the finale song called Carpe Diem. Everyone shows up including the people from season one and two.