Webby sees it as reflective of the existence of actual subterranean creatures
Launchpad wholeheartedly believes that there might be mole monsters around him and ends up ripping a poster into pieces
Mrs. Beakley sees the choice of film as indication of Lena being a negative influence
Webby and Huey's disagreement on the matter of subterranean creatures-notably the Terra-firmians-prompts Lena to lead them down into the city subway in search of the creatures. Mrs. Beakley, after dealing with a terrified Launchpad, notices their absence and quickly identifies Lena as the culprit. As the three youngsters make their way into a tunnel renowned by Terra-firmian sightings, the younger two Ducks each cite different sources: Webby's Diary and the Junior Woodchuck Guidebook. Despite his own experiences with the supernatural, Huey refuses to accept even the possibility of Terra-firmians' existing, blaming the subterranean activity they encounter on tectonic plates. The group eventually enters an abandoned subway train, where they are intercepted by Beakley and the others.
An annoyed Beakley tasks Launchpad with getting the train running so that they can get to safety; he takes Dewey, whom he suspects of being a mole monster, with him while Beakley takes Lena with her to detach train cars that are trapped by fallen rock. She quickly informs Lena that she has no intention of letting her have any contact with the children again, while Dewey is forced to deal with Launchpad's suspicions. Meanwhile, Louie looks on in amusement as Huey and Webby's argument continues, only for mysterious creatures to appear in the tunnel. Launchpad manages to get the train up and running, and Lena-after initially arguing with Beakley-helps her to get the train cars detached, only for Magica to appear in her shadow when the train's lights come on. Fully convinced that Dewey is a monster, Launchpad shoves him out of the driver car and starts the train, only for it to be hit by the creatures and be stopped by a rockslide.
Beakley becomes trapped under a train car, and despite Magica's suggestion that she leave the older woman to die, Lena uses her own powers to get Beakley to safety while she is unconscious. Dewey's attempts to reason with Launchpad at least convince the scatterbrained pilot that he's not a threat, while Huey is reluctant to leave the train car due to his fear of the unknown. However, Webby assures him that they'll figure it out together, and he accompanies her and Louie out of the car. After reuniting with the others, they run into the Terra-firmians only to find that they are not threatening monsters after all, and the creatures in fact guide them to a way out. As they are climbing out, Magica questions why Lena spared Beakley; when Beakley then expresses remorse for her previous judgment of Lena and extends a free invitation for her to visit the children in the future, Lena then tells Magica that this will get her closer to the McDucks.
When Webby is climbing a ladder halfway in, her tailfeathers are displayed purple instead of white for a frame.
While it's production number is 110, this episode was the fifth to be aired, having been rescheduled to air in October.
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. The red and pink Terra-firmians also have a conversation almost verbatim the same as their duck counterparts Huey and Webby.