Huey and Webby’s disagreement over the existence of a mythic species leads the kids to explore an eerie abandoned subway tunnel, while Mrs. Beakley grows suspicious of Lena.
At a movie theater, Huey, Dewey, Louie, Webby, and newcomer Lena finish watching a horror movie accompanied by Mrs. Beakley and Launchpad McQuack. While none of the kids were scared by it (sans Launchpad), each have their own opinions about the film; Huey dismisses underground monsters as an urban legend. Webby gets hysterical about Huey's disbelief toward the existence of subterranean creatures and claims that Duckburg has its own underground terrors: the Terra-firmians. To settle the debate, Lena convinces the two to check out the subway station nearby. While handling the kids and Launchpad, Mrs. Beakley notices their absence and quickly deduces Lena is behind it all.
As the three journey down the tunnel, Huey and Webby continue their argument on the real and the supernatural. Though he has encountered bizarre beings before, Huey refuses to accept even the possibility of Terra-firmians, and instead theorizes the movements underground to be from plate tectonics. The group stumble across an abandoned subway train, where they are ambushed by Beakley and the others.
Although irritated, Beakley instructs Launchpad to get the train started so they can get out of the tunnel while she and Lena detach the train cars that are trapped under rock; Launchpad, still traumatized, takes Dewey with him suspecting the latter to be a mole monster. Beakley then informs Lena that she will not let Lena continue seeing the kids.
Meanwhile, Huey and Webby's argument continues while Louie observes, however mysterious creatures begin to appear in the tunnel. At that moment, Launchpad gets the train running, but accidentally turns on the train's headlights. Due to brightness, Magica De Spell is exposed from Lena, who quickly turns away to hide her, and Dewey is blinded, which convinces Launchpad that Dewey is a mole monster. Launchpad pushes Dewey out of the driver car and starts the train, but the creatures hit it as it encounters a rock slide.
Beakley gets trapped and knocked out under a train car. Despite Magica's advice to leave the old woman, Lena uses her magic to lift the car and rescue Beakley. Dewey is able to convince Launchpad that he is not a monster, while the others are still inside the subway car, Huey is reluctant to leave the train car due to his fear of the unknown, but Webby assures him that they'll figure it out together, and he accompanies her and Louie out of the car.
The group reunites, but they are confronted by the creatures, who are confirmed to be the Terra-firmians. However, it turns out that the creatures are friendly, and they help create a passage to get the group to safety. As Magica questions her niece why she saved Beakley, Beakley apologizes to Lena and invites her back to the mansion; Lena claims this will get her closer to the McDucks.
- Kate Micucci as Webby Vanderquack
- Danny Pudi as Huey
- Ben Schwartz as Dewey
- Bobby Moynihan as Louie
- Kimiko Glenn as Lena
- Toks Olagundoye as Bentina Beakley
- Beck Bennett as Launchpad McQuack
- Catherine Tate as Magica De Spell
- Fred Tatasciore as Red Terra-Firmian/Theatre Manager
- Cree Summer as Pink Terra-Firmian
- This episode is loosely based on the Uncle Scrooge comic "Land Beneath the Ground!" by Carl Barks, which was also adapted as the 1987 episode, "Earth Quack". Unlike the comic and the 1987 episode, however, Scrooge and Donald do not appear in this episode, nor does Gyro or Duckworth. The great games are only mentioned and not shown, and the story takes place in an abandoned subway tunnel and not under the Money Bin.
- The Terra-firmians encountered by the group all share the same colors as Huey, Dewey, Louie, Webby and Lena.
- This is the second consecutively aired episode where Scrooge is absent.
- This is the third consecutively aired episode where Donald Duck is absent.