Chief Warrant Officer Sam Jacobs (Jack Kruschen), Dr. Iris Ryan (Nora Hayden), Professor Theodore Gettell (Les Tremayne) and Col. Tom O'Bannion (Gerald Mohr) are astronauts aboard the spaceship MR-1, the first mission to Mars. Shortly after their rocket reaches the mysterious red planet, the band of brave space explorers discovers monstrous yet highly intelligent alien life forms, which first attack, then deliver a haunting word of warning intended for all mankind.