American scientist Steve Karnes (Evans) delivers a speech to a British scientific society, headed by Professor James Bickford (Morell), about the dangers to marine life posed by nuclear testing. Before Karnes can return to the U.S., a real-life example of his concern materializes when a fisherman in Looe, Cornwall is killed on the beach, and his dying word is "behemoth". Later, thousands of dead fish are washed ashore.