Candace walks into the lair while Vanessa gets some tea. Candace then talks to Doof about her failure to bust her brothers, but Doof does not listen since he is looking for his nemesis. Candace then sits on one of Doof's inators, which he gets her off of. Doof explains that it is the Mixup-inator, which is able to go back to the first day of summer vacation. Candace is intrigued by this and believes that she might have a chance of busting her brothers. Without hesistatation, Candace activates the -inator, and a bubble appears surrounding the Earth.
The scene switches to the beginning of the first day of summer.
Baljeet and Irving are sitting under a tree, bored, wondering what to do. Baljeet realizes that they will be back in school shortly and they will have to say what they did over their summer vacation. He feels that their time off should be exciting, like a rollercoaster. Remembering the lame coaster they rode at the State Fair, Baljeet realizes that's what they should do today: build a rollercoaster.
As their mom, Betty Jo, heads off to do some shopping, Vanessa runs to her car to confirm that she's been put in charge. Mom doesn't think it's necessary that anyone be in charge, but Vanessa does. "What if a satellite falls out of orbit and crashes into the house?" It's not likely to happen, so her mom gives her approval and Vanessa rushes over to tell the boys, "Mom says I'm in charge! ...conditionally!"
That doesn't bother Baljeet. He and Irving have set up drafting tables and have started working on their "homework". Vanessa doesn't understand why they would be doing homework during their summer vacation. She heads into the house after admonishing them that they are being watched, and reminds the boys that she's in charge, conditionally.
Vanessa only gets to take one look out the window before the phone rings. It's Jenny, wanting to go to the mall with her. But she says she can't go because she has to watch her brothers. Vanessa makes sure that Jenny will see if Milo is there.
While Vanessa was talking on the phone, the boys and Patty have been transporting various building materials into the back yard: lumber, steel I-beams, a jackhammer and plumbing supplies. The tiger's roar breaks her concentration and she yells at the boys, "Will you hold it down? I am trying to use the phone!". Going back to talk to Jenny, she gets her first hint that something is wrong. Jenny tells Vanessa that she can see something being built in the backyard from her house.
Vanessa runs out to the backyard, then stops in her tracks, horrified at what she saw. She looks up and sees a patchwork of pipes and track. Vanessa shockingly asks Baljeet what it is, and Baljeet asks if she likes it. Whatever it is, she doesn't like it. She angrily storms off to tell Betty Jo, proclaiming that the boys are going down. She repeats it several times and spells it out for emphasis. "We're going to need a blowtorch and some more peanut butter" is Baljeet's assessment.
As Vanessa rides off on her bike, Suzy walks up to her asking if Baljeet is home, but Vanessa takes no notice of her and continues riding to the supermarket. Suzy however, walks into the yard with hearts in her eyes. "What'cha doin'?" she asks. Baljeet tells Suzy that they are building a roller coaster. Suzy then says hi to Irving, but Irving only waves his sledgehammer in response. As a man of action, he's busy laying track. He strikes a rivet with the sledgehammer, causing it to bounce and hit him on the nose. Irving just blinks, apparently unhurt. Suzy had stopped by to invite Baljeet to go swimming with her, but he wants to work on the coaster, so she heads off. Baljeet then notices that Patty is missing.
Patty appears, beats up some generic bad guys while Perry the Platypus & Spider-Men sings a medley of two songs . Then Patty gets his mission from Commander Carl. Commander Carl informs her that Professor Bannister has bought "80% of the country's tin foil". Agent P must find out why and stop him. Before Agent P leaves, Commander Car reminds her to maintain her cover as a "mindless domestic pet". Giving the Commander a quick salute, she flies off in her platypus-shaped hover-jet. She exits to the surface through a tunnel. Shortly thereafter, she flies by the boys and pulls her hat down over her face to avoid being recognized.
The boys are taking a break from construction to discuss the next part of the coaster. There will be solid-fuel booster rockets and snakes being released during a corkscrew around the Interstate highway. Baljeet heads off to get the snakes while Irving resumes welding.
Meanwhile, Vanessa has been trying to get Betty Jo to come home from the grocery store and see what Baljeet and Irving are doing. Betty Jo mentions that she recalls Vanessa doing the same thing previously, claiming that the boys were training "monkeys to juggle bicycles," but when she came home, "there was a stunning lack of monkeys". Vanessa persists, telling her about the roller coaster. Betty Jo thinks Baljeet is a little young to be a roller coaster engineer.
At that moment, Major Monogram at a car factory is asking Baljeet the same question. After Baljeet confirms it, the Major approves their building permit despite it being filled out in crayon. He offers to give them whatever help they need, so Baljeet takes him up on the offer and borrows one of the robotic assembly arms. In short order, the robot arm is mounted to the coaster cars and is laying out track for them.
Nearby, Agent P swings in through a window at Bannister's Evil Incorporated. Professor Bannister greets her with: "Ah, Patty the Platypus! What an unexpected surprise. And by unexpected, I mean completely expected!" After trapping Agent P, he informs Agent P of his plan to move the Eastern Seaboard (now covered in tin foil) to the West by using a "Magnetism Magnifier", which will reverse the rotation of the Earth. While Bannister is talking, Agent P tries getting free of the robot arms holding her, but then notices a screw on the ground at her feet. Looking around her, she formulates a plan...
Back at the Super Food Stuff Mart, Vanessa continues her attempts to convince Betty Jo to come home. She sees the poster that Irving has just put up, but when she gets Betty Jo over to there, two boys took the poster to see if they can get in for a discount. Vanessa screams when she sees that the poster is gone. Betty Jo informs her she'll be over in the dairy section if Vanessa wants to "come yell at some cheese or anything".
Baljeet and Irving unveil the Coolest Coaster Ever to the neighborhood kids. The first lift hill climbs up through the clouds, high enough that Pogo the Pig flies into the track and falls to the ground.
As they start to go up, Baljeet is showing everybody how the seat belts work, but he drops it. The coaster comes to a stop at the top of the lift hill, revealing part of downtown Danville far below them. He asks everybody, "You all signed the waivers, right?" They then shoot down and begin the wild ride. There are loops, hairpin turns, and the promised corkscrew. A load of rubber snakes falls on them just before they plunge into a mud bucket. Fortunately, the next section includes a trip through a car wash and they all emerge squeaky clean. Next is an "A-A-A-A" portion and a few loops around Bannister's Evil Incorporated.
Inside, Professor Bannister is still rambling on about how much this scheme cost him. Agent P uses her tail to fling the screw at Bannister, "missing" him, making it bounce off the control panel and hit him in the foot. Hopping around, he accidentally releases Agent P from her robotic arms. Agent P then starts to battle him with wrenches and strikes the same foot of Professor Bannister that the screw hit.
During the fight, Professor Bannister runs over to the Magnetism Magnifier and turns it on, telling Agent P she is too late and it cannot be stopped now. But the Eastern seaboard did not move to the west, just the tin foil, causing a two-ton ball of tin foil going, at two hundred miles an hour to head straight for them. They both frantically try to turn it off.
At the store, Vanessa and Betty Jo are at the checkout when Vanessa hears the coaster going by. She sees the coaster track out in the parking lot and goes back to get her mom so she can see as well.
Agent P is now helping Bannister in trying to move the Magnetism Magnifier, but it will not budge, so he hooks the magnet to a helicopter, causing the magnet to go with it. Professor Bannister is happy about being saved, but is then moved (along with the rest of the tallest part of the building) by the tin foil ball that crashes into it. The magnet attracts the Magnetism Magnifier, which sends a magnetic ray at the track under it, picking up the entire roller coaster.
Vanessa gets Betty Jo out of the store, but the roller coaster track has already been moved. Since Betty Jo has finished checking out, the two leave, and Vanessa is now excited because she believes that when they get home, the boys will not be under the tree, therefore proving to Betty Jo that she is right.
Agent P has been riding on top of the magnet and notices that the helicopter has been trailing a plume of smoke from the strain of lifting that much weight. She cuts the cable, making the roller coaster drop to the ground. The kids are still riding this very fast coaster. Agent P jumps onto the coaster with the kids as it passed beneath him. Baljeet looks behind him and utters the famous catch phrase, "Oh, there you are, Patty." Instead of seeing Agent P, Baljeet sees Patty again, since Agent P put her hat on Suzy a moment before, so she wouldn't be recognized.
The kids reach the broken end of the coaster and ride out into the street, through a construction site and up onto the tail of a passing jumbo jet after a crane swings them up into the air. Vanessa, in the car below, sees the boys on the airplane flying in the other direction and laughs gleefully. Betty Jo says she worries about her sometimes.
The coaster cars finally come loose from the tail of the jet and they fall onto the Statue of Liberty. It bends and then flings them over to Mount Rushmore, where they do some loops inside Theodore Roosevelt's glasses. They then shoot out on a tree over a Mr. Slushy Burger, then get flung to Paris where they land on a very bendable Eiffel Tower over a croissant stand, finally getting flung all the way into outer space. A satellite passes by the coaster, and Baljeet points out that if it falls to Earth, Vanessa is in charge. At that moment, the coaster plummets to the Earth, picking up speed and catching the nose of the first coaster car on fire from the friction of re-entry. As they get closer to Danville, Baljeet says, "We should've charged more."
Vanessa and Betty Jo now pull into the driveway and Vanessa runs out of the car, laughing crazily. She looks into the backyard, and does not see Baljeet and Irving and shouts "Yes!" She tells Betty Jo she told her, closing her eyes and extending her hand out to point at the yard, as we hear a rustle of leaves. Betty Jo says "Hi" to the boys, who are here, sitting under the tree. She tells Vanessa to help her with the groceries and walks away, leaving Vanessa to gasp "But" over and over again until Betty Jo pulls her into the house.
The kids now fall out of the tree, all saying how much they liked the ride. Suzy also falls out of the tree, telling them how great it was and asking them what they're going to do tomorrow. Baljeet says he doesn't know, and Suzy suggests teaching Patty some tricks. Baljeet says that platypuses don't do much, followed by Irving saying his first spoken words in the show: "They're the only mammals to lay eggs". Baljeet says maybe he'll lay an egg, and Suzy leaves, saying it really was the best coaster ever and the two make a great team. Baljeet replies, saying that he couldn't have asked for a better brother than Irving who then burps. Baljeet comments that he can smell the peanut butter.
Baljeet then asks what they should do tomorrow, and suggests making a list. Just then the top of the tree explodes, causing Vanessa to shout, "Mom!", who responds with, "Give it a rest, Vanessa!".