During World War I, a German U-boat sinks a British passenger ship and takes the survivors aboard. After getting lost in fog, the submarine surfaces near the unknown island of Caprona, a lost world where they find dinosaurs and Neanderthals.

The Doctor and Romana arrive on the marsh moon of Delta Magna in search of the next segment of the Key to Time. They are caught in the conflict between the native Swampies and the crew of a chemical refinery. The presence of a gun runner complicates matters; to make things worse, the Swampies intend to awaken Kroll, the giant god that lives beneath the swamps.

"James Compton is paid by wealthy Howard Townsend to photograph his mistress Simone . Intimate photo sessions lead to an affair. Simone is kidnapped and he is being blackmailed over the affair by the kidnappers. Some violent scenes."