The Champions are sent to investigate a tropical island where visitors have a history of disappearing. Coming on shore in two parties, they discover and thwart an international conspiracy to threaten world peace.

The Champions are assigned to escort an Eastern European head of state, Dubrovnik, on his visit to Britain. During a dive in a Scottish loch, their submarine is sabotaged and Dubrovnik is injured. Richard and Sharron are forced to face down a mutiny within the crew, who want to surface and save their lives; the submarine cannot be allowed to surface if Dubrovnik is to survive the surgery, which Sharron has performed.