The first sequence of the film introduces its American main character, Captain Dwight Towers (Gregory Peck). The U.S.S. Sawfish, a submarine, has survived the war. The ship was at sea during the attacks and has now made its way to the only known safe spot in the world. Captain Towers has brought the Sawfish into port and soon places the ship and its crew under the command of the Royal Australian Navy. There is no more U.S. Navy.
The captain’s Australian liaison is Lieutenant Peter Holmes (Anthony Perkins), a young married man with an infant daughter. Holmes and his pretty wife, Mary (Donna Anderson), decide to introduce Captain Towers to Moira Davidson (Ava Gardner), a beautiful rich girl who spends most of her time going from one party to the next, always drinking too much. Who better to entertain the captain and help him to not dwell on his lost wife and children? A romantic relationship begins to develop. Nonetheless, Towers continues to speak of his family as though they are still alive and that he has a future with them.
Soon an Australian admiral decides that the Sawfish should go on a mission to the North Pacific to visit the West Coast of the United States. At least some scientists are convinced that the Arctic region may be a safe environment. Only a trip there will tell. But the expedition finds that radiation levels in the Arctic Ocean are extremely high. San Francisco shows no sign of life. And a random beeping in San Diego turns out to be nothing more than a telegraph machine attached to a string on a window shade blowing in the deadly breeze. There's not one reason for hope.
Utterly dejected, the crew returns to Australia to live out their last few days. One of them, a scientist named Julian Osborne (Fred Astaire), competes in a deadly car race and takes the checkered flag. Dwight, who now accepts the loss of his family, takes Moira to a fishing resort where they spend a romantic night together. Within days, people begin to experience radiation sickness, and the public lines up outside of hospitals to receive government-issue suicide pills.
Back at home, the Australian lieutenant finds his wife beside herself. Once he is able to calm her down, the two of them share a tender moment. The crew of the Sawfish votes to return to the United States. Captain Towers would prefer to stay in Australia and die in Moira’s arms. But following the path of duty to the bitter end, he sails away with his homesick men, going down with the ship as it were.
To the extent that it takes up religious questions, On the Beach does so in an interesting way. There's a scene near the end—which, in this movie, means the conclusion of human history—that ironically portrays listless members of the Salvation Army conducting what amounts to an End-of-the-World service. (I'm guessing they didn't take up a collection). As if to suggest that humanity will never tire of its militaristic impulse, the band is heard to play “Onward Christian Soldiers.” Above the stage, strung between two poles is a huge banner that reads, “There is Still Time . . . Brother.” But the movie takes this appeal to prepare for the hereafter, and dramatically reinterprets it.
Before he boards the Sawfish for the last time, Captain Towers stands on the dock and holds Moira who tearfully says him, “There isn’t time. No time to love . . . nothing to remember . . . nothing worth remembering.” The message is clear: the Salvation Army’s banner appeals to a traditional religious understanding of reality that no longer commands assent. To temporal, sensual, truly-alive humanity, represented by all of the main characters, there’s no time for things like making love, sharing a meal, and delighting in children. Life, which means racing cars, going to the beach, drinking port, and growing old together, is no more. The emphatic ending of the movie shows the banner once more. This time the viewer knows its true meaning. Until a nuclear war breaks out, citizens of the world still have time to do whatever they can to try to prevent it. The unmistakable message of the film is that full-scale nuclear war will unspeakably and forever end the wonder of living. This, and not God, represents the good that human beings should treat as their ultimate concern. With the arrival of nuclear technology that has the potential to destroy human life, God has been upstaged. He no longer has the whole world in his hands. We do. And your brother? That's not your co-religionist, but every other person on the planet. Because of its unique dangers, the nuclear age requires the utmost in devotion and vigilance.