Marine atomic tests cause changes in the ocean’s ecosystem resulting in dangerous blobs of radiation and the resurrection of a dormant dinosaur which threatens London.

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.

An atomic bomb test in the Arctic Circle unfreezes a hibernating fictional dinosaur, a Rhedosaurus, that begins to wreak havoc in New York City.