Four people - Col. Tom O'Banion (Gerald Mohr), Dr. Iris Ryan (Nora Hayden), Prof. Theodore Gettel (Les Tremayne), and Warrant Officer Sam Jacobs (Jack Kruschen) - take off in a rocket on the first visit to Mars by humans from Earth. All goes well until they reach the strange planet some 47 days later and find they are anything but welcome. Donning space suits and helmets they explore Mars to discover that even the seemingly innocuous plant life is their vicious enemy. After several attempts are made by strange, horrifying creatures to kill them, they decide to return to Earth. The rocket ship will not budge, however, and they are shocked to see it surrounded by an enormous breathing amoeba, which they know will soon cut through the ship and kill them. They succeed in killing the giant amoeba at the cost of Warrant Officer Sam Jacob's life. They return to Earth, but Mohr has been attacked by the amoeba and will die if a cure is not found. All ends well with a message from Mars man forbidding Earth people to return to the planet.