Launchpad McQuack brings Scrooge to Gyro Gearloose's cabin, where the nutty professor offers the billionaire his newest 'newest invention' - Super Fizz, a fizzy drink that, according to him, "is great for your teeth". Scrooge and Launchpad don't seem to find it tasty, but Doofus Drake who happens to be there at the moment, thinks otherwise. With that method of foreshadowing out of the way, the group takes to the seas in a new submarine that Gyro has built especially for this episode, christened the Gold Digger. Scrooge recaps to them (and anyone who missed yesterday's show) the trouble Glomgold has caused for him in the previous episode. As our heroes dive down, Doofus reveals to Scrooge (and the viewers) that, by deflating balloons the right way, he can communicate with whales.
Shortly afterward, the quarter reaches the Marinara Trench and, thanks to the sub's metal detector, find a penny. After throwing another hissy fit, Scrooge figures that the rest of his fortune should be nearby, but in their attempt to follow the trail of the coins, they get caught in the current, which causes the Digger to bounce around like a pinball. And just when they're out of that, they're hijacked by large frogs, or as Doofus calls them, "fish folk", who apparently don't take kindly to air-breathers. The fish folks' leader, Master Malick, is angry about the water pollution that he blames McDuck's group for, and he sentences Scrooge's quartet to work in his city dumps for 400 years (or "only 100 years for each of us", as Launchpad puts it).
Oddly enough, the so-called dump is supposedly the long-lost city of Atlantis, and by some strange coincidence, this is where Scrooge's fortune has ended up. The ever-prejudiced frog-people won't let Scrooge collect it up, nor will they let him and his buds escape. As if that's not enough, the frogs have a Godzilla-like sea monster named Glubzilla as a guard! (And unlike last episode, it really is a sea monster.) Glubzilla has no interest in eating Scrooge's "ice cream", but he does destroy the Gold Digger, so our heroes' chances of escaping are just about nil. Instead of feeding Scrooge to Glubzilla, though, Malick's guards throw him in a room where the tide rises due to a leak in there.
In their own "hotel room", Gyro comes up with a new escape plan - getting the whole city of Atlantis to float using the Super Fizz from earlier! Doofus calls up Squeaker, his dolphin friend from earlier, and tells him to go retrieve some tires they need for the plan. Once Gyro combines all the ingredients, he sends Launchpad and Doofus' dolphin friends to go spread the goo all over the coral. Launchpad successfully performs his task, even despite Glubzilla ambushing him (it turns out that Glubzilla is very ticklish). Atlantis drifts up to the surface, with the fish folk not doing anything but curse Scrooge, demanding that he "come back with our trash", then realizing that the ducks have just done them a favor. Scrooge falls out of the flooding room where he was imprisoned to find Atlantis now on the surface and his cash back in his possession.