Description
In 1973, a nuclear-powered spaceship blasts off from Mars for Earth, bringing with it the sole survivor of the first mission, Col. Edward Carruthers (Marshall Thompson). He is suspected of having murdered the other nine members of his crew for their food and water rations, on the premise that he had no way of knowing if or when an Earth rescue mission would ever arrive. Carruthers denies this allegation, attributing his crew\'s deaths to a hostile humanoid life form on the Red Planet.
Commander Col. Van Heusen is unconvinced and makes sure that Carruthers is constantly accompanied by another member of his crew. While the ship was on the Martian surface, an emergency hatch had been left open, allowing the creature easy access. The crew are at first skeptical that something crawled aboard while they were on Mars. However, when Kienholz investigates odd sounds coming from a lower level, he is killed and his body hidden in an air duct. Next is Gino Finelli. He is found, barely alive, but the creature attacks his would-be rescuer. Bullets have no effect, forcing the crewman to leave Gino behind, much to the distress of his brother Bob. An autopsy of Kienholz\'s body reveals that it has been sucked dry of all fluids.
The crew use hand grenades and gas grenades, but the creature proves to be immune to both. They next try electrocution, also with no effect. When \"It\" is tricked into going into the spaceship\'s atomic reactor room, they shut the heavily shielded door and expose the creature directly to the ship\'s nuclear pile. It easily crashes through the door and escapes. The creature is so strong that it can tear through the metal hatches separating each of the ship\'s levels. The survivors (except for an injured crewman, who is trapped below in a spot inaccessible to the creature) retreat to the control room on the topmost deck. When Carruthers notices the ship\'s higher-than-normal oxygen consumption rate, he surmises that this is due to the creature\'s larger lung capacity, needed for the thin Martian atmosphere. In a last desperate move, everyone puts on their spacesuits, and Carruthers opens the command deck\'s hull airlock directly to the vacuum of space. A violent decompression follows, and the plan works: \"It\" suffocates and finally expires, stuck part way through the final hatch.
A press conference is later held on Earth, revealing the details of what happened aboard the rescue ship. The project director emphasizes that Earth may now be forced to bypass the Red Planet \"because another word for Mars is death\".
