Solana Beach, a seaside tourist resort, has come under attack by a giant octopus, which captures and devours human swimmers and boaters, picking the skeletons clean of flesh and bone marrow. Marine expert Will Gleason (Hopkins) and crusading newspaper reporter Ned Turner (Huston) suspect the construction of an underwater tunnel by the Trojan company, owned by Mr. Whitehead (Fonda). Trojan has been using radio signals that have been "above regulated levels", somehow maddening the octopus and causing it to attack human beings. In the end, the octopus is killed by Gleason's pair of killer whales.