While driving through the desert with his wife Carol Marvin to a military base to send the eleventh rocket into Earth orbit to assist the exploration of outer space in Operation Sky Hook, Dr. Russell A. Marvin and Carol see a flying saucer and accidentally records a message on their tape recorder. Once in the base, Dr. Russell is informed by his father-in-law and general that the ten first satellites mysteriously fell back to Earth. When Dr. Russell decodes the message, he encounters the aliens, who ask him to schedule a meeting with the leaders of Earth in Washington in 56 days in order to invade Earth without panicking the population. Dr. Russell develops an anti-magnetic weapon that becomes the last hope of the human race against the hostile aliens.

Jimmy Smigelski, living near the docks of Brooklyn, is quick to help when a neighborhood girl, Della, and her little brother are menaced by some thugs. Joe Brindo, a racketeer Jimmy's honest father Pete blames for an incident that crippled him, is impressed by Jimmy and takes him under his wing. Jimmy is caught in a rivalry between two local gangs.