AllMovie

Admiral Nelson (Richard Basehart) is aboard the newly-launched submarine Neptune, a sister-ship to the Seaview, for her shakedown cruise. The ship detects the presence of an unusual radioactive ore and the admiral and crewman Fowler go on a dive to investigate. Soon after leaving the ship, they see it approached, engulfed, and destroyed by a gigantic, jellyfish-like coelenterate. Making for the surface, the two men find themselves alone in mid-ocean on a sea filled with smaller versions of the same creature, which carry a painful and poisonous sting. Nelson makes a valiant effort to keep them both alive but Fowler succumbs during a storm that sweeps the area. Meanwhile, the loss of the Neptune results in an ocean-wide search for survivors under the command of Admiral Jiggs Starke (Harold J. Stone), an old friend of Nelson's but also a fierce martinet of a man. He gets the Seaview temporarily commissioned into the regular navy and comes aboard, intending to use it as his flagship -- but he immediately butts heads with the sub's captain, Commander Lee Crane (David Hedison), over the latter's more modern approach to command. While they square off over their philosophical differences, Nelson is rescued at sea. The sub makes a stop to pick him up, because he wants to return to the scene of the wreck and find the coelenterate before it can attack any other ships. Crane can't help notice, however, that Nelson is acting very strangely, at times even irrationally. His behavior -- which seems to match the symptoms that the ship's doctor (Richard Bull) has connected with fatal radiation poisoning derived from those smaller jellyfish -- and Stark's interference with Crane's command of the Seaview come to a head just as the sub's balast controls are jammed, and the coelentarate starts stalking the Seaview.