When an undersea volcano sends tremors throughout the Atlantic, aquatic creatures come out of hiding. Merchant seamen Joe Ryan (Bill Travers) and partner Sam Slade (William Sylvester) harness the most unusual of the lot, a massive beast that looks like a Tyrannosaurus Rex. The enterprising sailors deliver their quarry to a British circus, but it turns out that the animal is a newborn. And, when his much larger mother arrives in London to rescue her son, the city feels her wrath.