Scrooge McDuck and Flintheart Glomgold have been called to the city of Macaroon by the Grand Kishke, for he has discovered a cavern full of pear-shaped fruits that glow like lightbulbs. Since the Kishke isn't a businessman, he's decided to have the richest man in the world market the fruit, and he offers that Scrooge and Glomgold have a contest to determine which of them is richer by way of the giant scales he has outside. Glomgold then busts The Beagle Boys out of jail, offering to provide them with the weaponry they need to sneak into Scrooge's Money Bin.
The next day, the Beagle Boys arrive outside the Money Bin, accompanied by all their cousins, and begin their assault on the bin. Scrooge defiantly fights them off in many different ways, including shooting an armored Bigtime out in a tank. After the Beagles retreat, Scrooge installs a gigantic cannon in his vault and sets a trap for anyone who tries to sneak in. Unfortunately, he goes back in to get his cane, which triggers the cannon into shooting the bullet, which bounces back and causes the cannon to explode. The Beagles come back, seeing this as the opportunity to make their move, but they (and Scrooge) discover that because of the explosion, all the money has dropped into the sewers (hence this episode's title).
The Beagles head off to get some swimming gear, while Scrooge sends Huey, Dewey and Louie into the sewers to look for his fortune while he searches from above ground using a map of Duckburg's water system. Down in the sewers, the nephews make a boat out of an oil canister and manage to find the money and the blockade holding it. But thanks to the Beagles activating it, the blockade opens up, and the cash flows under the reservoir, taking the ducklings with it. Above ground, Scrooge heads for the pumping plant, where he finds the Beagle boys turning a valve. He fights back by turning the opposite valve, building up the water pressure below.
The money and nephews end up in a condo room, and they call up Scrooge to come in. But before Scrooge can have his cash shipped over to Macaroon, the Beagles reappear, claiming that nothing will stop them this time. Scrooge is too pooped and crazy to put up a fight now, but he manages to think up a new plan - dump the cash on the elevator the Beagles are using! It works. The Beagles, falling down the elevator, surrender and are taken back to jail.
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.
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.
Over in Macaroon, where there's only one hour left of the contest, Glomgold has already his own fortune onto the giant scales. To further ensure that Scrooge doesn't make it, he has sprung the Beagle Boys out of jail again to help him by using a super-sonar machine that should pick up the noises of whatever Scrooge is using to get here. Meanwhile, Scrooge, Launchpad, Gyro, and the nephews are up in the sky, right there on the now-floating Atlantis (held up by balloons), which is disguised as a giant cloud and being propelled by an exercise bike. Huey, Dewey & Louie are supposed to be tending to the cloud-making machine, but instead they've gone off to find the treasure of Atlantis so they can use it to help Scrooge win the contest.
Down below, the Beagle Boys catch sight of the floating Atlantis (because the nephews aren't tending to the machine, its cloud disguise is waiving), and Glomgold, who is patronizing with the Grand Kishke and contacting the burglars by way of a walkie-talkie hidden in his coat, orders them to take his airplanes and investigate. When the Beagles come up to attack, the nephews realize they abandoned their post and left the island open to attack. While Launchpad goofs around with a small Boeing that Gyro invented just for this episode, Scrooge chastises HD&L for the fact that he might lose this contest because of them. The Beagles try to shoot the balloons, but since Gyro made them steel-belted, they don't pop easily.
While Glomgold tries to make sure Kishke doesn't see the island, the Beagles try anchoring it in the other direction. The nephews catch sight of this and use a Wile E. Coyote-esque trick to get the island back on the right track, involving an extra balloon, a rope, and a boulder. It not only works, it gets the island moving scaleward faster because Gyro was pedaling on the exercise bike at top speed. Unfortunately, the sudden jolt causes Scrooge to fall right on him, causing the bike to break off, and it sends the both of them sailing off the island. Luckily, Gyro has brought along compact parachutes for them (and Launchpad) to use. They crash through Kishke's tent, and Scrooge picks a fight with Glomgold.
Up on the island, the nephews start steering the island themselves. When Scrooge sees this, he politely asks Kishke for some of the firefly fruits, which he uses to make a big glowing X on the right spot. Glomgold orders the Beagle Boys to try and stop the nephew trio. HD&L start untying the balloons so that Scrooge's fortune lands on his side of the scale. The Beagles bite back and release some balloons on the other side so that it falls onto Glomgold's side, but HD&L put up a big enough fight for Scrooge's scale to weigh more (Actually, Scrooge and Flintheart were tied at the point when there was no more money on the island - Scrooge used the Number One Dime as a tie-breaker). Refusing to lose now, Glomgold orders a tank nearby to bring in some previously unseen cash.
The nephews are then chased into a building by the Beagle Boys, where they find the fish depicted in a clue that leads to the treasure of Atlantis. They trick the Beagles into triggering one of the fishes while they trigger the other, causing the treasure to come pouring out onto Scrooge's scale. Scrooge's cash pile becomes so big it tips over Glomgold's scale and reveals that he was cheating - by using a lead bar so his scale weighs more. Scrooge wins the contest and congratulates his nephews for saving the day while Glomgold has to put up with Kishke wanting to buy his coat because he thinks it can talk.
The scene with the giant cannon is taken from the Carl Barks comic story "Terror of the Beagle Boys", which marked the gang's first appearance.
Using the Number One Dime as a tie-breaker is an homage to the Carl Barks comic story "The Second-Richest Duck" where Scrooge and Flintheart came to an agreement that whoever who keeps more string would be the Richest Duck and the tie-breaker was the piece of string that attaches the Number One Dime to Scrooge's pocket.
Scrooge and Glomgold having a contest to weigh their fortunes on giant scales was also used in the computer game DuckTales: The Quest for Gold.
The real estate agent from "A Drain on the Economy" also appears in "Magica's Shadow War".
When Scrooge grabs his accountant and pulls him onto the breakfast table, the tablecloth, plates and pancakes get shoved aside. In the next shot, the table is back to normal.
Goof: At the beginning of Working for Scales Burger asks Glomgold, " Oh, you got quiche?". But the DVD subtitles say "Oh, you got keys?"