Originally aired: Sunday January 18, 1970 on ABC
Writer: Bob Mitchell, Esther Mitchell
Director: Sobey Martin
Show Stars: Don Marshall (Dan Erickson), Deanna Lund (Valerie Ames Scott), Heather Young (I) (Betty Hamilton), Gary Conway (Captain Steve Burton), Stefan Arngrim (Barry Lockridge), Don Matheson (Mark Wilson), Kurt Kasznar (Commander Alexander B. Fitzhugh)
Guest Stars: Malachi Throne (Taru) , Joseph Ruskin (General Aza) , Peter Jason (Mylo) , Whit Bissell (Dr. Krane)
Production Code: 4719
While the Earthlings duck archaeologists and a mysterious saboteur who fires a gun that causes an explosion, Fitzhugh is wounded and the group takes refuge in a cave. It contains a teleporter which transfers Dan, Fitzhugh, and Valerie into a secret underground city. Magistrate Taru takes them into custody, and eventually believes their claim that one of his people were using a secret weapon to drive off the archaeologists despite laws against it. Taru is in an election against General Aza, who tries to poison Taru's drink. The little people secretly witness this and warn Taru. He declines to free Fitz and Valerie, but sends Mark to get help.
Mark comes back with Steve, and they come up with a plan to taint the drilling site with "omicrom 4", an explosive substance that will deter the archaeologists. They manage to do so when Betty causes a distraction and then gets caught (and subsequently freed). Taru wins the election, and an irritated Aza paralyzes Taru and then goes to the surface to start the war he has been trying to provoke.
In the subsequent struggle, Aza hits his head on a rock and dies, and the Earthlings get the weapon back. They unfreeze Taru and he sends everyone back, then destroys the teleport pad as the Earthlings are still at risk in the secret city, where their presence is illegal.