In 2526, eight Earth paleontologists and geologists were investigating a fossil found in a newly discovered cave when seven of them disappear. The only one who seems to have got out is Professor Kyle. She accompanies Lt. Scott and his troop of soldiers into the unmapped caves to see if they can find her colleagues. What they find, however, is the Doctor and his traveling companions Nyssa, Adric and Tegan, who have also just arrived in the TARDIS. They're near the location where the scientific expedition was attacked and Scott suspects the Doctor. There are others in the subterranean structures however.