While looking at items in Scrooge's office, Huey and Dewey notice Scrooge's "can" ringing. Scrooge and Donald explain it's from Donald's cousin Fethry calling from a top-secret deep sea lab to report an amazing discovery. They discourage the boys from answering as Fethry usually calls them down for an adventure that involves explosions and other catastrophic events only to show them something he found interesting, like a funny-looking barnacle formation. However, Huey and Dewey are attracted to the idea of going on an adventure with a scientist cousin and steal Scrooge's sub with Launchpad's help.
While Launchpad is inexplicably called off by a figure he knows as "Oceanika, (his love)," Huey and Dewey meet Cousin Fethry, who immediately strikes them as a bit eccentric. Fethry, who happily greets them as "Little Donalds," explains he is Donald's favorite cousin. He then takes them down in an aquavator towards the McDuck Sub Lab. Along the way, Dewey notes some kinship between Huey and Fethry, including their interest in bizarre facts and rather odd habit of observing their surroundings, which Huey grows insecure about over the course of the episode.
The aquavator's descent is slowed by a blast from some hydrothermal vents, so Fethry makes a pit-stop at a mid-descent station called the Tully Observatory to meet his "team." They are then attacked by a group of giant sea-worms, mutated from the toxic vents who can only be dissuaded from killing them through genuine displays of affection. Fethry and Huey are easily able to show the worms affection, but when Dewey is unable to come up with genuine compliments, Huey ultimately has to kiss one of the worms to get them to leave. Fethry then introduces his "team": a group of glowing krill in a jar of water that he's individually named.
They continue their descent towards the sub lab in the aquavator, where Dewey emphasizes Huey and Fethry are bizarrely similar, from the nerd-out gene, obsession with weird stuff, and even wearing a red that they don't take off. It is then revealed that Fethry is also a Junior Woodchuck and like Huey keeps his guidebook under his hat, which even weirds out Huey. Before they can continue, they realize that something has cut the cable to the bottom observatory, forcing the group to don diving outfits to reach Fethry's private pod. To do this they have to swim through the field of hydrothermal vents. When the light from the aquavator shuts off, Fethry sings to his krill, causing them to light up and guide their way to the pod.
Upon reaching the pod, they learn the truth of Fethry's discovery: a group of glowing multi-colored krill that he spotted floating outside the porthole. When the boys are, of course, disappointed, Fethry is shocked to realize they thought he was a scientist, and reveals he's just the custodian for the lab. The boys are prepared to angrily depart when a giant monster appears outside the porthole, a discovery which satisfies Dewey.
They quickly swim away from the monster through the field of erupting hydrothermal vents, which gradually destroy the rest of the lab. They then make it back to the aquavator to start their ascent but are stopped by the continued eruptions which cause the aquavator to start flooding. Huey and Dewey use the aquavator's light to try to blind the monster, only for Fethry to realize the monster is Mitzy, one of the krill on his team, who mutated into a gigantic size from exposure to the toxic vents. Dewey continues trying to blind Mitzy, leading to a struggle between him and Fethry. Realizing Mitzy is trying to help them, Huey turns on the exterior speakers and sings Fethry's song to Mitzy, which elicits a reaction and she calms down before helping bring the deteriorating aquavator back to the surface. Encouraged by the adventure, Fethry decides to go and become a scientist for real (although he is unsure how to do so), and thanks the boys before having Mitzy take him away. Huey remarks that Fethry is very weird, to which Dewey assures him there is nothing wrong with that.
They return to the sub just as Launchpad returns decked out in oceanpunk gear and bidding "Oceanika" goodbye. Huey then directs them all to head home so they can convince Scrooge "this was Louie's idea."