Huey and Dewey are led on an adventure by their distant cousin Fethry Duck, who takes them to the depths of Scrooge's deep-sea laboratory.
While observing items in Scrooge's office, Huey and Dewey notice Scrooge's "can" ringing. Scrooge and Donald explain it's 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 happily greets them as "Little Donalds" and casually explains that he is Donald's favorite cousin. He 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 and memorizing the Junior Woodchuck Guidebook, about which Huey grows insecure over the course of this 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 attacked by a group of Giant Sea Worms mutated from the toxic vents. The worms can only be dissuaded from killing them through genuine displays of affection. Fethry and Huey are easily able to show the worms affection and give them compliments, but when Dewey is unable to come up with genuine compliments, Huey ultimately is forced to kiss one of the worms to get them to leave Dewey alone. Fethry introduces his "team": a group of glowing krill that he has individually named in a jar of water.
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 hat they don't take off. 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, swimming 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 frustrated and 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 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 lullaby to Mitzy, which elicits a reaction; 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. He thanks the boys and has Mitzy take him away. Huey remarks that Fethry is very weird, but Dewey assures him there is nothing wrong with that.
They return to the sub just as Launchpad returns decked out in oceanpunk gear, bidding "Oceanika" goodbye. Huey directs them all to head home so they can convince Scrooge "this was Louie's idea."