In 1964, nuclear war wipes out all of humanity in the northern hemisphere.
One American nuclear submarine – the USS Sawfish – was submerged at the time of the nuclear interchange and finds temporary safe haven in Melbourne, Australia, which is one of the only places in the world not yet affected by radiation.
In denial about the loss of his wife and children in the Holocaust, American Commander Dwight Lionel Towers (Gregory Peck) is befriended by Royal Australian Navy Lieutenant Peter Holmes (Anthony Perkins), who is a family man with a wife and newborn baby, Jennifer.
Towers also meets careworn but gorgeous Moira Davidson (Ava Gardner), who begins to fall for him.
Picking up an incoherent morse code signal originating from the US Pacific Coast the Sawfish heads out to sea again – taking drunkard scientist Julian Osborne (Fred Astaire in his first straight dramatic part) with them – and sails thousands of miles to find out if anyone is still left alive in the United States.
They finally surface in San Francisco Bay, where the eerie sight through the periscope of an empty and dead city is too much for San Francisco native seaman Ralph Swain (John Meillon), who jumps ship and swims to shore.
The final glimmer of hope is dashed when Lt. Sunderstrom (Harp McGuire) goes ashore in San Diego, protected by a radiation-proof suit, and discovers the source of the mysterious morse code signal – an empty coke bottle attached to a window blind, tapping the morse code clicker.
The Sawfish and her crew return hopeless and aware that Australia and the rest of the mankind have very little time remaining. The radiation is coming.
The local population come to terms with their impending fate as one might expect: denial, anger, clinging to the thinnest hope and, finally, resignation as they begin to plan for their own deaths before the deadly radiation starts to take effect.
Mary Holmes (Donna Anderson) has a breakdown because she cannot bear to put her young daughter Jenny to sleep forever. But both she and her husband Peter eventually come to terms with the end by having little Jenny drink her formula, spiked with a lethal dose of sleeping pills.
Then in a last and emotional embrace, they have a drink of hot tea with cyanide pills mixed and dissolved in it.
Julian Osborne has no problem with the thought of his own demise and locks himself in his garage where he starts the engine of his beloved champion racing car and asphyxiates himself.
Commander Towers and Moira say a tearful farewell (“It’s been nice, Dwight Lionel. It’s been everything”) and he takes the Sawfish and his crew out to sea and back home to the USA, to die alongside their already deceased families and loved ones.
The US Department of Defence and United States Navy declined to cooperate in the production of the film and would not provide access to a nuclear-powered submarine. This forced the film production to use a non-nuclear, diesel-electric Royal Navy submarine, HMS Andrew.
Cmdr. Dwight Lionel Towers
Lt. Peter Holmes, Royal Australian Navy
Lt. Cmdr. Farrel