After a conversation between Marlin and Dory, the Moonfish unexpectedly appear. In an effort to entertain Dory, they begin doing various "impressions," which consist of them moving into different positions to form various shapes including a swordfish, a lobster, and a sailing ship. They try to get Dory to identify the subject of each of their impressions, and are constantly annoyed when Marlin gives her the answers.
Once Dory explains to them that Marlin lost his son Nemo, the Moonfish decide to help by providing directions. They begin doing impressions to illustrate the route to Sydney, where Nemo has been taken. They also tell them to swim through the "trench" (actually a submarine canyon) not over due to the jellyfish living above it but they swim over it anyway due to the trench having death written all over it due to Marlin's fears.
One of the members is known as Ted; he has relatives in Sydney.
They aren't seen again for the rest of the film.
- When the Moonfish do their "impression" of a sailing ship, they sing the chorus to A Whale of a Tale, a song performed by Ned Land in Disney's 1954 classic 20,000 Leagues Under the Sea.
- The Moonfish also appear in the teaser trailer for Finding Nemo, forming the basic shape of a fish and an arrow.
- The Piston Cup Precision Drill Team from Cars are described as Nemomatic MenV's. Because the drill team cars often work together to form letters and words, the car make is probably a reference to the moonfish from Finding Nemo.
- Given that the Moonfish warn Marlin and Dory to swim through the trench and not over it, they probably dealt with the jellyfish that live up there themselves.
- The groupers that Nemo freed from the fishing net in Sydney Harbour look similar to the Moonfish.
- The Moonfish leader's voice actor, John Ratzenberger, later voiced a crab named Bill in the 2016 sequel.