After an armoured car transporting radioactive material inexplicably caught fire, the driver managed to crash the vehicle into the sea, temporarily sealing the radiation beneath the surface and buying time for the authorities to come up with a solution.
However, unbeknownst to anybody, the high level of radiation had mutated a number of ocean creatures into a single monstrous humanoid Sea Beast, which came ashore over the next couple of nights, killing and eating five people, including a tramp camped out on the beach, and a man taking a dip in his swimming pool.
Local Police Inspector Coal and scientist Professor Ross soon deduced the existence of this monster, and waited on the beach one evening for the Beast to emerge. Their plan was to blind the creature with a powerful light before Inspector Coal shot it with his service revolver. The plan was partially successful, and whilst they did manage to blind the Beast, its armored hide proved to strong for the bullets to penetrate. After smashing the light, the Beast received an electric shock, which caused it to retreat back into the ocean.
Meanwhile, Doctor Tower, another scientist working on the project, had developed a dispersal agent to be used on the radioactive pollution, but due to its nature would have to be taken deep beneath the surface by a human diver. This task fell to Professor Ross, but while he was delivering the serum, encountered the Sea Beast. After a brief struggle, the professor managed to inject the creature with a syringe full of the agent, causing it to de-mutate into the harmless creatures it had evolved from.
In the aftermath, a young boy out swimming swallowed some of the residual contaminant left in the ocean, and was seen in the last panel beginning to change into something grotesque . . .