USS Charleston (SSN-704), a nuclear-powered submarine equipped with a caterpillar drive, remains on station after a devastating nuclear exchange, under the command of Dwight Towers [Armand Assante].
A global confrontation between the United States and China escalated into a war that contaminated the northern hemisphere, following a standoff that saw China blockade and then invade Taiwan. The conflict leaves much of the world decimated, and the Charleston’s crew seeks refuge in Melbourne, Australia, where the radioactive fallout has not yet arrived. Radio operators farther north warn that radiation is spreading, and Australia will face the threat in the months ahead. In response, Towers places the submarine under the Royal Australian Navy and is summoned to a briefing that includes a curious automated digital broadcast coming from Alaska. The mission is laid out: investigate the broadcast with Julian Osborne [Bryan Brown], an Australian scientist, and Peter Holmes [Grant Bowler], an Australian liaison officer, aboard.
Upon reaching Alaska, Towers and his executive officer land to find no survivors. Inside a house, they discover a dead family, and the sight unsettles Towers as he contemplates his own family’s possible fate. The source of the broadcast turns out to be a solar-powered laptop attempting to relay a documentary via satellite. During the mission, the executive officer sustains a punctured suit and a leg injury, but keeps the injury hidden from Towers.
Rather than return directly to Melbourne, the Charleston heads to San Francisco, the origin of many crew members. The Golden Gate Bridge lies in ruins along the shoreline, and at least one crewman, thinking of his homeland, deserts the ship. His shipmates leave him behind, convinced that radiation poisoning has already condemned him. Back in Melbourne, the executive officer’s health deteriorates into terminal radiation sickness, and Towers fulfills his dying wish by euthanizing him, a quiet moment of friendship amid the catastrophe. Towers then reconnects with Moira Davidson [Rachel Ward], Holmes’s sister-in-law and Osborne’s former fiancée, as the city braces for the approaching cloud.
As Melbourne faces the inexorable advance of radiation, social order begins to crumble. People line up for government-provided suicide medication, and families confront illness and loss. Osborne races across the Phillip Island Grand Prix Circuit, culminating in a fiery crash at Turn 10. With most of the Charleston’s crew suffering from advanced radiation sickness, they request one final voyage to San Francisco, hoping to die together aboard the only home left to them. Towers grants the request, seemingly leaving Moira Davidson behind as the submarine sails away. Although not shown on screen, the ending implies that Towers and Moira will later take suicide pills together, bound by a fate impossible to escape and a bond that remains after all else has fallen.
