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.

Gene Evans
as Steve Karnes

André Morell
as Prof. James Bickford

Leigh Madison
as Jean Trevethan

John Turner
as John

Alastair Hunter
as Dr. C. A. Morris, the village doctor

Lloyd Lamble
as Senior Admiral, at the Admiralty

Jack MacGowran
as Dr. Sampson, the paleontologist

Maurice Kaufmann
as Mini Submarine Officer

Henri Vidon
as Tom Trevethan

Patrick Jordan
as Photo Lab Aide