An atomic war has seemingly destroyed most (if not all) human civilization, leaving Earth contaminated with radioactive fallout. The apparent single exception is a box canyon, surrounded by lead-bearing cliffs, in which former Navy Commander Jim Maddison lives with daughter Louise (Lori Nelson) in a home stocked with supplies anticipating just such a holocaust. Louise is engaged to be married, but her fiance is missing.
Into this natural bomb shelter come stumbling survivors, who by chance were inside the canyon when the 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, billed as "Touch" Connors), with his "moll" Ruby (Adele Jergens), 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, before the rain comes to wash what's in the atmosphere down to Earth, contaminating the shelter. The second threat comes in the form of a giant, hideous monster, which seems bent on killing anything it comes across, but only eating 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 the two women to himself.
All three dangers coincide as the monster kidnaps Louise. It then releases her into a small lake, where he is obviously afraid to enter. Rick appears and attacks the creature, which runs away as it begins to rain. Following the creature destroyed by the rain, they realize that it is Louise's missing fiance. Tony, having stabbed Ruby to death when she realizes that he wants to be with the younger Louise, then steals Jim's pistol. He waits to waylay Rick when he returns with Louise. As he takes aim, Jim reveals a concealed pistol and shoots Tony dead.
Jim then dies from radiation poisoning, but not before revealing that the rain is radiation-free and will wash away all of the remaining contamination, making the world safe to venture out into again. He has heard voices of other survivors on the radio. Rick and Louise, the final two survivors of this group, walk hand-in-hand out of the canyon, with the end-card "The Beginning" appearing on the screen.
Who shoots Tony dead?
Jim