The Flynn-Fletchers take a cruise while Doof steals shrunken goods.
Plot[]
At the Danville Docks, the Flynn-Fletcher family board a cruise ship in hopes of being able to see a natural phenomena that only comes around once a decade, the Tri-State Aurora. Unfortunately, no pets are allowed on the ship, so they left Perry to be petsit by Stacy, who is still fully aware of his secret identity as Agent P. While playing video games with Stacy, Agent P is informed by Major Monogram that Doofenshmirtz will be on the same ship and to stop his shenanigans since his vaguely European aunt Olga is celebrating her birthday on that ship. Stacy promises to cover for him while he is away.
Doof and Vanessa check into their stateroom on the ship and Doof shows his daughter his latest creation, the travel-size-inator, which shrinks anything down to travel size so he can fit it in his fanny pack and take it home with him. Vanessa is concerned that he is stealing from the ship, but her father insists that the brochure says the cruise is "all-inclusive", so he is not breaking any rules.
When the Flynn-Fletchers board the ship, Lawrence is about to get a historical tour, but gets caught in a conga line, Linda invites Candace for a relaxation session at the spa, and Phineas and Ferb hang at the kids' club and help a couple of kids improve their fortress made of building blocks.
Perry approaches Doof, who accidentally makes Aunt Olga stub her toe, and their fight takes them to the ship's buffet. Over the loudspeaker, the ship's captain informs the passengers that the Aurora may not be visible from their vantage due to unforeseen cloud coverage. This gives Phineas another idea of what to do onboard the ship. Candace feels a disturbance, but her masseuse assures her it is only the massage oil.
While touring the ship, Vanessa bumps into her old flame, Monty Monogram, who is on the ship with Aunt Olga, who is soaking her toes in a foot bath. Aunt Olga gives Monty permission to hang with Vanessa at the buffet. During their fight, Perry and Doof accidentally shrink various things, before shrinking themselves and they end up in the potato salad. As Monty and Vanessa grab some food, Monty accidentally grabs Doof and Perry as well.
Back at the kids' club, Phineas and Ferb decide rather than just make the clouds disappear, they will use excess balloon animals from two party clowns to make the ship soar above the clouds so everyone can see the Aurora.
Back at the buffet, Monty and Vanessa eat and chat about their estranged relationship, which Doof finds out about. Vanessa assures him they are in international waters and not legally star-crossed lovers anymore. Monty unknowingly puts Doof and Perry in his mouth with his next bite of potato salad. Perry slingshots some potato salad at Monty's uvula to make him cough them out. As the disgruntled busboy discards Monty's napkin with Doof and Perry on it out the window, they are suddenly captured by a seagull.
While the kids enjoy the Aurora, the seagull drops Doof and Perry into Aunt Olga's soak tub, as Doof starts a sword fight with Perry using plastic sabres and tiny umbrellas. At the spa, Candace overhears Phineas congratulating Ferb, then escapes her relaxed state. While she is impressed with the Aurora, she goes into busting mode, and drags her mom out of her chair while she still has her mud mask and cucumber slices over her eyes. During their tiff, Perry and Doof end up in Phineas and Ferb's building block fortress, as Perry knocks Doof backwards, firing up his inator making the balloon animals attached to the ship's funnel shrink, causing the ship to descent back to the water. Perry also causes Doof's inator to shrink all the clouds out of the sky making the Aurora fully visible, just in time for another failed bust attempt.
Doof suddenly remembers that his inator had a reverse switch. He activates it, causing not only himself and Perry to grow back to normal size, but also all the things that Doof stole from the ship. Doof and Perry are then caught and arrested by two of the ship's security officers, but only because Doof supposedly broke the "no pets allowed" rule by bringing Perry onboard, despite the fact that Perry isn't his pet and that he went on his own. It also turns out that the cruise's all-inclusive clause gave Doof the legal right to steal from the ship (as pointed out by the officers), causing a distraught Perry to realize that he battled Doof just for nothing. As such, a frustrated Doof and Perry end up being handcuffed and hauled away by the officers.
Everyone aboard the ship, including Monty, Vanessa, and the Flynn-Fletchers marvel at the Aurora; even the officers allowed the handcuffed Doof to admire the Aurora as well, not knowing that Perry has escaped and replaced himself with a balloon replica.
Phineas and Ferb's idea to inflate enough balloons to make the cruise ship float is similar to Up. Where in that movie, Carl Fredricksen utilizes many balloons to make his house float.
