Phineas and Ferb Bikini Bottom Vacation

While on a cruise, the Flynn-Fletchers are attacked by a giant wave, which scatters the passengers across the Pacific Ocean. Phineas, Ferb, Candace, and Isabella drown to the bottom of the ocean, however PnF quickly create a substance that, when swallowed, allows one to breathe underwater. They land in Bikini Bottom, where they meet up with characters from SpongeBob SquarePants to do various activities. Meanwhile, the adults are desperately trying to find their children.

Act One
The Flynn-Fletchers are seen about to leave on a cruise. As they are about to board, Isabella rushes up to them, stating that her parents have said she can go with them. The group gets on the cruise, and their vacation begins. A song plays as a montage of the characters doing various activities is seen. The montage stops at what is presumed a few days later. The group is seen in their cabin, sleeping. Rumbling sounds are heard, and Ferb wakes up. He taps on Phineas and Isabella's backs, waking them up, and tells them of the sounds he heard. The trio go outside their cabin to the edge of the ship and spot a huge wave off in the distance. Intrigued, they go to tell the captain.

Notified of this, the captain alerts the cruise and tries to get help through radio. However, his efforts are fruitless, and the passengers are soon looking up at a 50-foot wave. They scramble back to their cabins, awaiting the final crash. Phin, Ferb, and Isa hide in a separate cabin - being the one they could find the fastest after all the other cabins were taken - and hold each other tight as the wave crashes. The cruise is literally broke apart, and passengers float in the water. Phin, Ferb, and Isa drift in the water. Phin and Ferb find one of their tool kits and quickly concoct a substance that, when swallowed, allow them to breath underwater, and the trio take it.

While falling down into the water, the trio spot Candace, unconscious. They rush to her, and feed her the substance. Candace comes to, and upon seeing the trio, thinks of them to have something to do with her in the middle of the ocean. Once Phin explains about the wave and how they saved her life, Candace is a bit more grateful. For now, the group can only descend... and descend...

Meanwhile, in Bikini Bottom, SpongeBob SquarePants has strayed from his route to the Krusty Krab and is walking in unfamiliar territory. He is somewhat frightened, and he then hears a loud "THUD!" behind him. He hastily turns around and cautiously approaches the thing that fell. Coming closer to it, he finds that it is Phineas, Ferb, Candace, and Isabella, asleep. The group begins to open their eyes. SpongeBob dashes behind a rock to safety. The group gets up and wonders where they are. Ferb then spots someone behind a rock and slowly approaches him.

SpongeBob backs up into the darkness. Ferb beckons him to come out. SpongeBob slowly emerges from the darkness, showing his face to an astonished crew. Candace asks who he is, and SpongeBob questions who they are, feeling insecure. Phineas introduces the crew. SpongeBob warms up to Phineas and introduces him and tells of where they are. The group is surprised at this, and SpongeBob offers to give them a tour of the town, which they agree to.

Meanwhile, the rest of the passengers have washed up on a desert island. Linda & Lawrence are distressed that they have lost the kids, and shout "NOOOOO!!!" as the camera pans out, then fades to black as dramatic music plays.