In the second episode of the four-part story "Earthshock," the Doctor (Peter Davison) and two of his companions are held responsible when four 26th century paleontologists disappear. The actual culprits turn out to be a brace of blank-faced robots who are jealously guarding a giant bomb within a huge cave. Clearly, the robots are acting under someone's orders -- but whose? Written by Eric Saward, "Earthshock, Episode 2" originally aired on March 9, 1982. ~ Hal Erickson, All Movie Guide