When an undersea volcano sends tremors throughout the Atlantic, aquatic creatures come out of hiding. Merchant seamen Joe Ryan and partner Sam Slade 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.