An atomic war has seemingly destroyed most, if not all, of human civilization, leaving the Earth contaminated with radioactive fallout. The apparent single exception is an isolated box canyon, surrounded by lead-bearing cliffs, in which former U.S. Navy Commander Jim Maddison (Paul Birch) lives with his daughter Louise (Lori Nelson) in a home that he has stockpiled with supplies in anticipation of such an apocalypse. Louise is engaged to be married, but her fiancé is missing.
Into this natural bomb shelter stumble survivors, who by chance were inside the canyon when the atomic war occurred. After initially refusing to admit them, Jim relents when his daughter appeals to his humanity. Among the survivors are a geologist, Rick (Richard Denning), who happens to specialize in uranium mining, and a small-time hood, Tony (Mike Connors) with his "moll" Ruby (Adele Jergens), who were on their way to San Francisco.
There are two struggles for survival: the first is a simple question of whether the radioactive fallout will dissipate, and if so, if it will do so before the rain comes to wash out what is in the atmosphere to fall to Earth, contaminating the shelter. The second threat comes in the form of a hideous atomic mutated monster (Paul Blaisdell), which seems bent on killing anything it comes across, but only consuming those creatures contaminated by fallout.
A less obvious but no less dangerous threat is the hidden menace of Tony. Although seemingly charming and helpful, his true character and intentions are that he wants the other men out of the way, so that he can have both women for himself.
All three dangers coincide as the mutated monster kidnaps Louise. It then releases her into a small lake, where it is obviously afraid to enter. Rick appears and attacks the creature, but it runs away as it begins to rain. Following the creature as it is being destroyed by the rain, they come to realize that the creature is Louise's missing fiancé. Tony, having stabbed Ruby to death after she realized that he wants to be with the younger Louise, then steals Jim's pistol. He quietly waits to ambush Rick when he returns with Louise. As Tony takes aim, Jim produces a second pistol and shoots Tony dead.
Jim has been slowly expiring from radiation poisoning. He reveals that the rain is radiation-free and will wash away all of the remaining contamination, making the world safe to venture out into again. As he dies, Jim also reveals that he has heard voices of other survivors on the radio. After the rain, Rick and Louise, now the final two survivors, walk hand-in-hand out of the canyon (as the end card "The Beginning" appears on screen).
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