With a week left of the contest, the nephews' head over for Scrooge's new ice cream factory for free samples, right next door to the Bin, but are denied access. They become suspicious of the trucks coming from out of said factory. That night, the boys look out their window and see the ice cream trucks passing by. Still suspicious, they sneak out and hitch a ride onto the trucks, where they discover that the drivers are supposedly stealing Scrooge's money and automatically loading them onto a ship. It turns out that Scrooge is actually loading his money by ship and not by airplane. As he explains this to the boys, neither of them notice a suspicious-looking spy camera nearby...
The next morning, Scrooge is informed that one of the boats carrying his cash was attacked and swallowed up by a sea monster near the Marinara Trench. After Scrooge throws a temper tantrum as a result, he and the boys go diving to find the remains of the ship, discovering that it's being quarantined by the Navy. When the Navy divers won't let them pass, they use a giant turtle to get in and find that the ship is empty. Scrooge pins the blame on the Navy, who don't take kindly to that and take them to the ship that Donald happens to work on.
Despite Scrooge's threats, Admiral Grimmitz denies having Scrooge's cash. After Scrooge leaves the room, the Admiral calls in Donald, because he might know the culprit - Dr. Horatio Bluebottle and the SS Moby, a top-secret super-sub he invented for the Navy. Donald's job is to monitor Uncle Scrooge, while the Navy keeps track of his progress with a little tracking device on Don's back. Shortly afterwards, Donald joins Scrooge, who's riding along with the second portion of his cash, and they discover the so-called sea monster, which is actually the SS Moby disguised as a killer whale. In either case, it swallows up the second boatload of the money, taking Donald and Scrooge with it.
Inside, they discover that the German-accented Dr. Bluebottle's in cahoots with Glomgold, who's ordering him to keep the cash down there till after the weigh-in. Donald and Scrooge corner him, and a fight ensues. Eventually, Bluebottle has them tied up (like any good German villain) and prepares to shoot them out his torpedo tubes. But before he can, the Navy starts dropping bombs on his sub, because thanks to Donald's homing device, they knew where to fire. Unfortunately, Bluebottle pulls it out and squishes it, leaving the Navy to be sitting ducks (no pun intended).
Fortunately, Donald and Scrooge manage to untie themselves and smash the steering mechanism while Bluebottle is too busy fighting with the Navy to pay attention. The doctor loses control of the steering, and his sub is blasted by one of the Navy's bombs, causing it to sink into the Marinara Trench. That is, until Scrooge pulls the eject lever, releasing his money into the trench and catapulting the sub back up to the surface. Donald gets the Silver Tar, Bluebottle goes to prison, and Scrooge's fortune is out of Glomgold's grasp.