Candace's favorite girl band, "The Bettys" is having a radio contest, the winner of which will get to join them on their tour bus to their next concert, one that Candace and Stacy fell certain that they will win, given their clear obsession over the band. Fate is on the girls' side, it seems, as the two win the contest, leaving Phineas and Ferb on their own to decide between three possible activities.
Suddenly, Phineas notes Perry's absence, wondering where he could possibly go off to when he disappears, but as he ponders the subject, he unknowingly activates the secret entrance to Perry's hideout located in the tree, seating Phineas and Ferb in Perry's chair. Phineas, though slightly confused at first, assumes it's Ferb who built it all, meaning that they will choose the plan to become secret agents for the day. As Phineas is congratulating Ferb, who is about to mention that he didn't actually build any of it, Major Monogram burst onto the screen, though reading a piece of paper, thus, not looking at the boys, as Perry himself comes down in an elevator and attempts to get the Major's attention without notifying the boys.
After reading what Phineas clearly takes as a script he was given, Monogram suddenly realises that he isn't speaking to Perry and quickly gets off screen, asking Karl who they are. As soon as Phineas and Ferb take off, Monogram quickly assigns Perry his mission, not only to stop Doofenshmirtz, but to protect the kids, as Perry uses his chair to pursue, what with Phineas and Ferb having taken the hovercraft and all.
While Phineas and Ferb marvel in the features of the hovercraft, including the cup holders, down on the ground, Candace and Stacy are somewhat enjoying helping The Bettys make their new song, by not only cleaning up the entire bus and shredding all of their fan mail, including a letter sent from Candace herself, and cleaning the outside of the bus, nearly getting killed in the process. By the time Candace has finished all of these tasks, she begins to show her irritation, which finally bursts as the Bettys request them to clean the toilet, bringing with their request a speech from Candace about the proper way to treat one's fans.
Meanwhile, up in Doofenshmirtz' new lair, Perry has arrived before Phineas and Ferb to stop Doofenshmirtz, however after a quick battle including a vicious automatic tennis ball machine, Perry noticed the boys incoming. Quickly grabbing Doofenshmirtz, Perry hid in the closet as Phineas begun to point out all of the things that 'Ferb' had done to the evil lair, finishing at the destruction ray, which had been set to a timer in the battle prior. The machine fires off it's laser while Ferb still attempts to explain that he never didn't make any of the things, destroying a bridge that is right in the way of The Bettys tour bus.
None of the passengers take notice, however, as Candace and Stacy have inadvertently helped the band write a new song, one that causes the driver to come in to the back of the bus and congratulate them. Only when he brings Candace and Stacy to the front to see the autopilot system installed in the bus do they realise the danger they are in, as the driver faints and all of the brakes break.
Luckily, Phineas and Ferb have noticed this, and with the help of the hovercraft's tractor beam and Perry's guidance from underneath the bus, the band and the girls all make it to the other side of the bridge safely, earning new respect for the girls from The Bettys, who believe them to have saved the day. After a bit of joking around, the band members agree to let Candace and Stacy sing onstage with them at the next concert.
Meanwhile, as Phineas and Ferb congratulate themselves on an awesome day achieved, they suddenly lose control of the hovercraft, finding themselves turned upside down by Perry and dropped into Isabella's pool, shocking more than just one of them. As the boys return home, Phineas' far too excited explanation of why he and Ferb are wet confuses Linda and causes Ferb to give up attempting to point out that he never built any spy mission.
The episode ends with Candace and Stacy singing onstage with The Bettys.