Aging, respected Captain Paul Blanchard (Heston) is on his final submarine tour before promotion to command of a submarine squadron (SUBRON). Surfaced and returning to port, the submarine, USS ''Neptune'', is struck by a Norwegian freighter enroute to New York in heavy fog. With the engine room flooded and its main propulsion disabled, the ''Neptune'' sinks to a depth of 1,450 feet (442 meters or approx. 241.6 fathoms) on a canyon ledge above the ocean floor. A United States Navy rescue force, commanded by Captain Bennett (Keach), arrives on the scene, but ''Neptune'' is subsequently rolled by a gravity slide to a greater angle that does not allow the Navy's Deep Submergence Rescue Vehicle (DSRV) to complete its work.