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 they swallow a substance that allows them to breathe underwater. They land in Bikini Bottom, where they meet up with characters from the TV show SpongeBob SquarePants to do various activities. Meanwhile, Linda and Lawrence are desperately trying to find the kids.