One morning in Duckburg, Huey, Dewey, Louie, and Webby are getting ready to go fishing. Meanwhile, Scrooge is at the Money Bin, waiting for two locksmiths to finish installing a new lock for his vault, when the kids show up. The kids invite Scrooge to go fishing with them, but he refuses their offer as he is too busy.
The kids go fishing at the docks, and only manage to catch old boots. While they’re fishing, they notice that many of the local sailors have pets of their own, and that they’re happy. Webby then gets the idea to find a pet for Scrooge, in hope that he’ll be more joyous. Conveniently for them, a sailor named Barnacle Biff is being instructed to let go of his pet lemming, as the critter eats too much. Biff overhears the kids talking about a pet, and offers to sell his lemming, whose name is revealed to be Lucky, to them. Biff tells the kids that Lucky is not a huge eater, and is smart, so they take him.
Over at the Money Bin, the locksmiths finish installing the new lock, give Scrooge the only copy of the combination, and leave. Scrooge puts the note in a locket, and places the locket on a statue of Glittering Goldie. He intends to memorize the combination after he eats his lunch. At that moment, the kids walk into the office and present Lucky to Scrooge. Scrooge can’t stand Lucky right from the start, as the lemming eats away at his sandwich. Lucky then runs around the place, knocks over the Goldie statue, and the locket gets caught around his neck. To make things worse, Lucky escapes by jumping out a window.
Since Lucky has the only combination, and because Scrooge hasn’t learned it yet, the group needs to find Lucky immediately. Duckworth drives Scrooge and the kids through Duckburg after Lucky, who returns to the docks, and boards his old ship, the Moony Gull, just as it leaves port. Scrooge demands for the local sailors to tell him where the Moony Gull is headed and they refuse to tell, until Webby directs their attention to all the old boots that she and the boys caught earlier. They tell them that the Moony Gull is travelling to Novay, a country in Scandinavia.
Scrooge gets Launchpad to fly him and the kids to Novay by helicopter. Upon reaching the shore, Launchpad crashes into a lighthouse. Fortunately, they got there just as the Moony Gull docks. Scrooge boards the ship and asks the captain if he’s seen Lucky, which leads to Biff explaining that he tried to get rid of him, only for him to return. Wanting his locket back, Scrooge accidentally scares Lucky, who runs down to the cargo hold, only to find his way back up. Launchpad and Scrooge both try to catch Lucky some more, but they both fail, and Lucky escapes by sailing away in Scrooge’s hat.
The kids search around the local town for Lucky, only to learn that the cheese merchant is fleeing town, and he lets them take a large cheese wheel. Scrooge finds his hat on the beach, but Lucky isn’t in it. On top of that, everyone is fleeing town, shouting "The lemmings are coming! The lemmings are coming!" And indeed, hundreds of lemmings come stampeding through the town, eating up any food they come across. To protect their cheese wheel, the kids suspend it above the ground with some rope, and hide atop a large haystack, only for the lemmings to eat most of the hay.
Scrooge asks a local man what exactly is going on, and he learns that every year the lemmings come running through, eating everything they can on their way to the sea, and swim away forever. The group tries to come up with an idea to stop the lemmings and find Lucky, and Launchpad suggests spraying them with water from a fire truck, but it doesn’t work. Huey suggests that they dress up in winter gear to scare the lemmings, but it only humors them. Scrooge begins to get carried off by the stampede, but Huey and Louie save him with a tree branch just before the lemmings jump off a cliff and into the sea.
After all the lemmings leave, Scrooge is saddened that he’ll never get the combination back now. Before deciding to go home, the kids suggest having some of the cheese that they hung up earlier. Scrooge could care less, but the kids discover that the locket, as well as Lucky, were both inside. Scrooge wonders why out of all the lemmings that only Lucky was in the cheese, and Dewey explains that Biff told them that Lucky was smart for a lemming, thus he was able to reach the cheese.
Later on, back at the Money Bin, Scrooge is counting his money in the vault, when the kids show up, this time with another pet. Scrooge doesn’t like the idea at first, but he quickly changes his mind upon seeing what they got him; a goldfish.
- One of the locksmiths appears as a mover in the episodes "Time Teasers" and "Dr. Jekyll & Mr. McDuck".
- DuckTales: Volume 2