Milo, Melissa, Zack, and their classmates from Jefferson County Middle School enter a subway station in order to travel to the Natural History Museum. While enroot, however, the car that the three enter becomes detached from the rest of the train and ends up going down the wrong track. Surviving the mishap, the trio encounter the Undergrounders, a group of construction workers who have been lost in the underbelly of the city for about a month. The group have formed their own society they have dubbed Subterranis, where they live among various leavings of surface civilization and eat rats.
At the museum's gift shop, the teacher is agonizing over the absence of Milo and the others while Bradley Nicholson states the fact to Mort that Milo's absence is allowing him to get more attention, though he fears it cannot last. Back underground, the trio and the Undergrounders are found by Diogee, whom Milo realizes can lead the group home due to his uncanny sense of direction. They then follow the dog only for Milo and Melissa to become trapped in a cave-in, forcing Zack to go to their rescue. Diogee then leads the group to an abandoned subway car that takes them into the museum, reuniting Milo and his friends with their classmates. Most of the Undergrounders then leave to return to their normal lives, though Scott opts to return to the underground.
For a full transcript of "The Undergrounders", click here.
The image gallery for "The Undergrounders" may be viewed here.
On September 19, 2016, the episode became available for free streaming on iTunes and Amazon.
On September 26, 2016, the episode became available to watch early on the Disney XD App and Video On Demand.