In Solana Beach, California, a baby named Jane disappears from a coastal park. Elsewhere, a local man is mysteriously pulled from his boat in the marina and his body later pops up, stripped of all tissue. That night, Sheriff Robards wants to keep the investigation under wraps to prevent people from jumping to conclusions. However, one of his officers wonders if the deaths are related to an underwater tunnel construction project, and veteran journalist Ned Turner suggests a looming catastrophe. Ned works all night on his story and in the morning, his sister, Tillie, expresses her concern for his health. Later, Mr. Whitehead, the president of Trojan Inc., the company digging the tunnel, reads Ned’s story in the newspaper and asks the project manager, John Corey, if there is anything connecting the company to the deaths; Corey denies the claim. At the coroner’s office, a medical examiner confirms to Robards and Ned that both skeletons were stripped clean, but he has no idea how it happened. When Ned suggests bringing in marine biologist Will Gleason, Robards tells him that Gleason has already sent two divers to explore the tunnel site. Later, Ned visits Gleason at his research facility, where he has developed a special bond with killer whales while studying them. Ned asks why Gleason did not personally investigate the deaths and learns that the marine biologist is recovering from a case of decompression sickness, also known as the bends. Meanwhile, Gleason’s divers discuss Corey’s attempted bribe to keep them from inspecting Trojan’s operation on the seafloor. Their dive is later thwarted when they are attacked and killed by a giant octopus. That evening, at a restaurant, Ned confronts Corey, who refuses to be interviewed. Ned learns that Corey is there to meet with Will and his wife, Vicky. Will tells Ned that the deaths of his divers have made the investigation personal for him. Later, at their hotel, Will promises Vicky that he will not dive deeper than 150 feet. The following day, Ned’s sister Tillie registers her son Tommy and his best friend, Jamie, for an upcoming open-water yacht race. Meanwhile, Robards and Ned discuss what they know about the four deaths and Ned points out that there was radio usage prior to each fatality. During a dive, Will and his assistant, Mike, investigate the scene and discover dozens of dead fish on the ocean floor. Afterwards, Will hypothesizes that Trojan’s illegal use of high-pitched frequencies is wreaking havoc undersea and a giant octopus may be responsible for the deaths. Elsewhere, Vicky’s sister, Judy, and two friends, Don and Chuck, get lost on a sea cruise, where they are attacked and killed by the octopus. Later, Ned calls Whitehead for a reaction to the marine commission findings that attribute the deaths to the tunnel project; Whitehead warns Ned to get a good lawyer if he is going to make baseless accusations. That night, Vicky and two men search for Judy’s boat and become victims to the octopus. The next day, Whitehead reigns in Corey, who admits that he has pushed Trojan’s tests beyond legal limits to save time and money. At police headquarters, a grief-stricken Will shares his findings with Robards and Ned, and Ned asks if Will can kill the octopus. As the junior yacht race begins at the harbor, Robards orders the danger zone to be expanded beyond thirty miles and the race is halted. A Coast Guard helicopter warns the boaters to return to shore, but most of the boats capsize in the high winds and the octopus pulls Tommy and Jamie’s boat under; however, two boats rescue the boys and return them to shore. Tommy is safe, but Jamie is not among the survivors. Later, Will and Mike enlist the help of their killer whales, Winter and Summer, to track down the octopus. As their ship rocks and the whales appear to leave, Will and Mike don their scuba gear and enter the water. There, the octopus strikes. Will is pinned beneath rocks and loses his tank, but the whales return and attack the octopus while Mike shares his oxygen with Will and guides his friend to the surface. The whales continue fighting the octopus until they destroy it. Back aboard the ship, Will assumes the whales have left for freedom and tells Mike he is thinking of going on safari in Africa. However, Summer and Winter return to the ship and serenade the men.