Hank McHenry and Johnny Marshall work as power company linesmen. Hank is injured in an accident and subsequently promoted to foreman of the gang. Tensions start to show in the road crew as rivalry between Hank and Johnny increases.

Fate brings a job at Boulder Dam and romance with a saloon singer into the life of a young man on the run.

Lt. Col. Glenn Manning is inadvertently exposed to a plutonium bomb blast and although he sustains burns over 90% of his body, he survives. Then he begins to grow, but as he grows he starts losing his mind. By the time he stops he is 50 ft tall, insane and is on the rampage.

The plot revolves around a rivalry between sand-hog "Hard Rock" Harrigan (O'Brien) and his foreman Black Jack Riley. At the center of their conflict is their mutual affection for heroine "Andy" Anderson (Irene Hervey). But when the chips are down and Riley is trapped in a tunnel cave-in, it is Harrigan who comes to the rescue.