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 Beagles 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.