A POP arrives in the middle of a tennis game between Fishtronaut, Marina, Zeek, Billy, and Mac, giving them an antique key and a slime-covered anchor. The twins think the clues must have something to do with the Blue Pirate's buried gold. On this mission, they discover, with Dr Green's help, that the real treasure is the pirate's historical documents and maps they found.
Fishtronaut's cousin, who looks just like him, decides to pay a visit. While Fishtronaut is resolving a technical problem with Rosie's automatic lipstick applier, his cousin borrows his Bublex and explores the park on his own. Everywhere he goes, he's mistaken for the real Fishtronaut, causing all sorts of confusion throughout the park. The situation gets suddenly serious when the Bublex's dripometer goes off and he doesn't know what to do. It's a rescue mission for Fishtronaut, Marina, and Zeek.