In the Salton Sea, an underwater earthquake causes a crevice to open, releasing prehistoric giant mollusks. A rescue training parachute jump is conducted, but the patrol boat sent to pick up the jumper finds only a floating parachute. One sailor dives in but also disappears. The other sailor screams in terror as something rises from the water.
When the patrol boat does not answer radio calls, Lt. Cmdr. John "Twill" Twillinger takes a rescue party out on a second patrol boat to investigate. They find the deserted patrol boat covered in a strange slime; the jumper's body then floats to the surface, now blackened and drained of bodily fluids. Twill takes a sample of the slime to the base lab for analysis, where he teams up with recently widowed Gail MacKenzie and Dr. Jess Rogers.
A young couple disappear after going for a swim. U.S. Navy divers investigate and discover a giant egg and the body of one of the victims on the ocean floor. The divers are attacked by a giant mollusk (which looks like a giant caterpillar), which kills one of the divers. The mollusk attacks the boat, but Twill stabs it in the eye with a grappling hook. The egg is taken to the U.S. Navy lab for study and kept under temperature control to prevent it from hatching.
The mollusks escape into an irrigation canal system, attacking livestock, a lock keeper, a trysting couple, and others. Navy divers locate a group of mollusks in the canal system, and use explosives to destroy them.
In the meantime, Gail is at the lab with her young daughter, Sandy. Worried about the lab rabbits being cold in the lab's lowered temperature, Sandy surreptitiously turns up the thermostat. Twill calls the lab and gets no answer. He arrives and finds that the hatched mollusk has Gail and Sandy cornered in a closet, where they ran to escape from the monster. He fights it with lab chemicals and a CO2 fire extinguisher, and a live steam line until other Navy personnel arrive and shoot the mollusk.
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