Plot summary

The title and credits are shown over a night view of the Salton Sea in California. We open with an aerial view of the Salton Sea (daytime). The narrator tells us, "This is the Salton Sea in Southern California. A strange phenomena in which nature has placed 400 square miles of salt water in the middle of an arid desert. In the desert close to shore of the sea the government has established one of its most important Naval Research Bases. At the laboratory on the secluded south tip of the base top secret atomic experiments are carried out under rigid security controls." An earthquake rocks the base. A few hours later the base if back to normal, but under the Salton Sea a fissure has opened. A parachute jump is executed over the sea. The recovery team in a patrol boat is waiting to pick up Lt. Hollister. Seamen Fred Johnson (Jody McCrea) and Howard Sanders (William Swan) spot the plane and steer towards the jumper. All they see when they arrive is the parachute. Johnson dives into the water to search for Hollister. Johnson surfaces, then dives under the boat to get to the other side. He does not resurface, but Sanders is confronted by something we only briefly see as a shadow that causes him to recoil in horror. He screams. The land radio operator, Seaman Wyatt (Charles Tannen) tries to re-establish contact with the patrol boat. Unsuccessful, he calls Naval Intelligence. Sally (Wallace Earl credited as Eileen Harley) answers the call. Wyatt eventually talks to Lt. Cmdr. John "Twill" Twillinger (Tim Holt). He and Lt. Robert "Clem" Clemens (Harlan Warde) take a boat out onto the sea to investigate. They spot the deserted boat and pull up along side. The body of seaman Sanders is on deck. He is dead. There is no sign of the other crewman or the parachutist. Twill notices a sticky white residue on the port side of the boat. He directs Clem to, "Look at this stuff." They take a sample. Hollister's body surfaces and they notice that his skin is dark and his eyes bugged out. He has a shriveled appearance.They return to base. Clem hands Twill the sample. Twill takes it to the lab for analysis. Gail MacKenzie (Audrey Dalton is typing a report and Dr. Tad Johns (Max Showalter credited as Casey Adams) is pulling a file when Twill enters. Dr. Johns gets Dr. Jess Rogers (Hans Conried). They enter the lab and Dr. Rogers starts his work and asks about Lt. Hollister's body. He is told the body was shriveled. Twill asks if radioactivity could have anything to do with the accident or the condition of the body. Dr. Rogers assures him it did not.Connie Blake (Marjorie Stapp) enters the office with Sandy MacKenzie (Mimi Gibson), Gail's daughter. Connie is pregnant and about seven months along. Sandy asks her mother if she can go look at the rabbits--research animals. Connie is there to collect her husband, George Blake (Dennis McCarthy), a lab assistant. Dr. Rogers tells Twill that the white substance is a simple marine secretion, but he wants to run a few more tests.Twill visits the County Morgue. He meets with Coroner Nate Brown (Byron Kane) and Sheriff Josh Peters (Gordon Jones). The coroner tells Twill the cause of death on Hollister was a puncture wound. The body was drained of blood and water. The coroner is at a loss to explain how. As for Sanders, it was a stroke, caused either by extreme excitement or fear. The sheriff quarantines the beaches. Back at the lab, Twill calls to speak to Dr. Rogers. He wants the report as it is their only clue. George Blake notices the white substance is radioactive.Mrs. Simms (Sarah Selby) operates a diner. She and her daughter, Jody (Barbara Darrow) disagree with Jody's choice in men. They argue and Jody leaves to meet with her boyfriend, Seaman Morty Beatty (Robert Benevides) on the beach. They go swimming. Mort dives under the water and does not resurface. Jody screams and is pulled under the water by something. Twill and the Sheriff meet with Mrs. Simms as both have disappeared. Outside Twill see two children fight over a sailor's cap. Twill questions the little boy (Charles Herbert) about the cap. He takes Twill to the place where he found it. Twill discovers the seaman's uniform and Jody's belongings. Mrs. Simms sobs uncontrollably. They also find more of the radioactive white substance on the rocks.Dr. Johns and George Blake make their first dive in the sea but find nothing. They dive again. At 60 feet they take a radioactivity reading, and everything is normal. At 125 feet they are at the bottom of a canyon. They see an object. It is a large sphere, white in color and billowy like a balloon. It is radioactive. They cut it loose and send the marine growth to the surface. Lurking below is a two eyed creature with pincers around its mouth. Before they surface they find the shriveled body of Jody Simms. The creature attacks and kills Blake. Dr. Johns escapes and swims for the surface. Safely on board, Johns tells them about the creature. The creature surfaces and attacks the boat. Twill pokes it in the eye which sends it back to the bottom howling.Connie arrives at the dock, unaware her husband is dead. She jokes with Gail, Dr. Johns and Dr. Rogers. Noting the expression on their faces, she asks about her husband. Dr. Rogers takes her to the radio shack to explain. Gail hangs back and tells Twill how she reacted to the death of her husband, Tom, two years earlier. Dr. Rogers and Twill join the others at the boat for a briefing. They intend to kill the creature with depth charges. Back at the lab, Dr. Rogers briefs the Navy brass on the egg they are incubating. He explains they control its growth by keeping the water temperature at 38 degrees. He explains the creature is a mollusk. He runs a film of other mollusks, mostly snails. Photographic evidence suggests at least ten other creatures exist and must be destroyed. Dr. Rogers is concerned that if even one reproductively active creature that gets into the All-American Canal, "The species could threaten the entire world." The Navy proposes to beef up the local sheriff's staff with their own to patrol for the creatures. Twill asks Gail about Connie. He suggests dinner, but Gail thinks it is too soon. Connie is staying with her during her grieving period. Patrols are now underway and Twill's office is a buzz of activity. Gail changes her mind about dinner. They dine at a Mexican restaurant on the border. A patrol spots an abandoned car on the canal cut-off road. They search the area and find a man in distress. He mumbles incoherently about "Mary Jean" then collapses. He is covered with the white foamy substance. Clem tells Twill about another report of cattle shriveled about 15 miles away from the abandoned car incident. Dr. Rogers deduces there must be an underground river connecting the Salton Sea to the canal system. He further suggests an old survey map could be helpful. Helicopters are now added to the vehicle patrols.Twill stops at the Imperial County Museum. He speaks to Lewis Clark Dobbs (Milton Parsons) about an old map, but they find nothing. The lock keepers check in with the base to report any sightings. Lock 57 answers a telephone call and reports nothing. Then outside, the keeper hears something splashing in the water. It turns out to be nothing but two boys splashing in the canal.Gail is working late. Sandy is bored and waits for her mother to talk on the telephone to steal into the lab to play with the rabbits. Sandy adjusts the temperature on the incubating egg because she thinks it will warm the room for the rabbits. Mother and daughter leave the lab. The watchman at Lock 57 (Ralph Moody) again hears something outside. He goes to investigate assuming the kids have returned. The creature sneaks up and attacks the watchman, killing him. When Lock 57 fails to report, a car is dispatched. The deputies find his body and call in. Dobbs stops in to talk to Twill, but just misses him. When Twill returns Dobbs meets with him and gives him an old map. The next morning Twill and Dr. Rogers take a helicopter to look for a small lake. They spot one with white foam floating on the surface. They've found what they are looking for. The Navy returns with divers and depth charges. Twill will be diving with Dr. Johns. In the lab, the water is warming up and the creature is close to hatching. The divers spot several giant snail-like shells on the bottom. The creatures are dormant. The divers plant charges near the shells. They start their ascent. They make it to shore as the explosions happen.Twill calls the lab to talk to Gail, but the line is busy. Sandy arrives in the lab and her mother is on the phone. She sneaks back into the lab and finds all the rabbits dead, their cages torn apart. The creature roars its presence. Sandy cries out for her mother. Gail bolts across the room, grabs Sandy and locks herself in a closet. The creature begins to chew through the door. Twill and Dr. Rogers arrive at the base and drive to the lab. They walk in and see the creature pushing against the door. Twill employs a CO2 fire extinguisher to spray the creature. It reacts by directing its anger towards him. That gives Gail and Sandy a chance to escape the closet. A high pressure steam hose seems more effective. That gives Dr. Rogers a change to get some guards with weapons. Three guards fire rifles, filling the creature full of lead. It falls, collapsing and breaking a lab table. Twill carries Sandy out of the lab. Gail walks beside them. Twill tells Sandy she can go swimming again. He puts Sandy down and we close with the three walking towards the camera away from the lab building.