In the seaside town of Solana Beach, California, a shocking event unfolds when a young baby named Jane goes missing from a local coastal park. The community is further shaken as a local man mysteriously vanishes from his boat at the marina, only for his body to later surface, horrifyingly stripped of all tissue. That fateful night, Sheriff Robards (played by Claude Akins) is determined to keep the investigation low-profile to avoid causing panic among the townsfolk. However, one of his officers and veteran journalist Ned Turner (portrayed by John Huston) begins to suspect that these strange deaths may be tied to a nearby underwater tunnel construction project, leading Ned to warn about a potential catastrophe lurking just beneath the surface.
As Ned tirelessly researches the mysterious circumstances surrounding the deaths, his sister Tillie (depicted by Shelley Winters) expresses her deep concern regarding his health and well-being after he dedicates a long night to gathering information for his story. The next morning, Mr. Whitehead (played by Henry Fonda), the president of Trojan Inc., the company responsible for the tunnel project, reads Ned’s unsettling article and inquires with project manager John Corey (embodied by Cesare Danova) if there could be any connections between the company and the horrendous events. Corey promptly dismisses the notion.
At the coroner's office, a medical examiner reveals the chilling conclusion to Robards and Ned that both skeletons were found completely stripped clean, yet the baffled examiner cannot explain the phenomenon. Ned suggests consulting marine biologist Will Gleason, which Robards disregards, noting that Gleason had already sent divers to investigate the site. Later, when Ned visits Gleason at his research facility, he finds a man deeply connected to the marine world, particularly with the same killer whales that he has been studying. However, Will explains that he cannot participate in the investigation due to his recovery from decompression sickness, a serious condition he suffered while diving.
Complications arise as Gleason's divers uncover Corey’s attempt to bribe them to avoid inspections of Trojan’s underwater operations. Tragically, their dive ends in catastrophe when they are attacked by a colossal octopus, a harbinger of the dangers stemmed from the tunnel project. Meanwhile, during a dinner confrontation, Ned faces off against Corey, who adamantly refuses to answer any of Ned’s questions, leading Ned to learn about the personal stakes involved for Will after the deaths of his divers. As Will resolves not to dive below 150 feet in the aftermath of these tragedies, events continue to spiral.
Ned’s sister, Tillie, prepares for an upcoming yacht race for her son Tommy and his friend Jamie. At the same time, Ned and Robards continue to piece together clues linking the tragic deaths. The duo uncovers alarming details involving radio usage right before each incident. During one critical dive, Will and his assistant Mike stumble upon a graveyard of dead fish on the ocean floor. Will hypothesizes that Trojan’s unlawful implementation of high-frequency sonar waves is wreaking havoc underwater, potentially inviting the monstrous octopus to wreak havoc on the unsuspecting community.
As the darkness of the situation grows, Judy, Vicky’s sister, embarks on a sea cruise with friends Don and Chuck, only to meet a gruesome fate at the hands of the very creature that embodies the town’s nightmares. Meanwhile, Neds confronts Whitehead about the marine commission’s findings, only to be warned to prepare for potential legal action should he continue on his current path of accusations against Trojan. That night, a desperate search ensues as Vicky and two men attempt to locate Judy's missing boat, but soon they too fall prey to the ravenous octopus.
As tensions rise, Whitehead holds Corey accountable for pushing Trojan’s limits to achieve faster results. At police headquarters, Will, stricken with grief, reveals his findings to Robards and Ned, raising the critical question of whether they can halt the octopus’s rampage.
As the junior yacht race kicks off, Robards orders an immediate expansion of the danger zone, ultimately canceling the event. Despite the Coast Guard’s warnings, disaster strikes when an unexpected storm capsizes many boats, leading to a heart-stopping moment when the octopus drags Tommy and Jamie’s boat under the waves. Fortunately, two other boats return Tommy to safety, but tragically, Jamie is not among the survivors.
Determined to put an end to the chaos, Will and Mike activate their trained killer whales, Winter and Summer, in a daring effort to tackle the octopus. As Will and Mike dive into the depths, the octopus attacks violently. With Will pinned beneath the rocks and his oxygen supply severed, the whales heroically return to assist, ultimately helping Will and Mike to the surface. In an adrenaline-fueled showdown, the whales unleash their fury upon the octopus, leading to its ultimate destruction.
Back on the ship, Will reflects on the ordeal and contemplates an adventurous safari in Africa, believing his dedicated companions have left in search of freedom. But to his astonishment, Summer and Winter reappear, faithful as ever, serenading the duo and ensuring that their incredible bond remains intact, even in the face of such extraordinary challenges.
