While exploring Scrooge's study, Huey and Dewey are alerted by a call from one of Scrooge's hotlines. Scrooge and Donald inform them the caller is from their cousin Fethry Duck stationed in Scrooge's underwater laboratory. Though their uncles express irritation of dealing with Fethry, the boys enthusiastically answer the call and coerce Launchpad McQuack to navigate them down without Scrooge knowing.
Huey and Dewey are both excited to meet Fethry, Huey hoping to learn from him and Dewey hoping to find exotic creatures. The group arrives outside the lab—a lighthouse—where Fethry is in the observation deck waiting. Huey and Dewey rush upstairs to meet him as Launchpad docks the sub when he hears a familiar voice calling to him coming from a girl named Oceanika. Inside, the boys are given an eccentric introduction from Fethry. He takes them down from the lighthouse on the aqua-vator to the McDuck Sublab for a tour. On the way down, the aqua-vator experiences turbulence from the ocean's hydrothermal vents. Fethry has the aqua-vator stop at the Tully Observatory for repairs, but the group is ambushed by giant sea worms mutated by the hydrothermal vents. Fethry instructs the boys to embrace the sea worms and show them affection, allowing them to escape the worms' clutches. There, Fethry retrieves his "team" of bioluminescent krill to help light up their way the deeper they go.
Something cuts the cable of the aqua-vator and the group is forced to go the rest of the way scuba diving. They enter Fethry's personal pod and learn he has been living in the pod for four and a half years. Fethry reveals his discovery: rainbow lights that he can observe from his window. Fethry also reveals he isn't a scientist but the lab's caretaker, prompting Huey calling Fethry out for wasting their time, crushing Fethry's spirits. Right as Huey and Dewey prepare to leave, the rainbow lights are revealed to be emitting from a gigantic monster. The group retreats back to the aqua-vator with the monster in pursuit. The inside begins to flood from damage, so Huey and Dewey shine light on the monster to scare it away. Fethry recognizes the monster as Mitzy, one of the krill that belonged to him and mutated to colossal size. Dewey still tries to blind her against Fethry's concerns until Huey decides to take Fethry's advice and sings a song that calms Mitzy down. Dewey puts his trust in Huey and he lowers the light, enabling Mitzy to clutch the aqua-vator and deliver it to the surface. Fethry bids farewell to the boys and departs with Mitzy to become a scientist. The boys run into Launchpad returning from his own underwater adventure. The group then heads home to convince Scrooge their excursion was Louie's idea.
- David Tennant as Scrooge McDuck
- Danny Pudi as Huey Duck
- Ben Schwartz as Dewey Duck
- Tony Anselmo as Donald Duck
- Tom Kenny as Fethry Duck
- Beck Bennett as Launchpad McQuack
- Jennifer Hale as Oceanika
- This episode marks Fethry Duck's animation debut.
- Fethry working at an underwater facility, and the fact that he's voiced by Tom Kenny, might be a nod to the titular character from a Nickelodeon TV series of the same name.
- When Fethry first appears in person, he's actually holding a drinking glass with a sponge in it.
- 1964, the date that the McDuck Sublab of the Future was created, as mentioned by Fethry, is likely a reference to his very first print appearance in the Donald Duck comic The Health Nut, drawn by Al Hubbard and written by Dick Kinney and first printed in Italy in Topolino #453.
- The tune to which Huey and Dewey sing, "Best time waste ever!" sounds nearly identical to the titular refrain of Grease's "We Go Together".
- Launchpad going on an "off-screen adventure" previously occurred in "The House of the Lucky Gander!", right down to having the same wistful reaction in the end.