The film opens with a classic 1950s version of a spaceship (three tail fins, long, pointed body) perched on the cratered surface of an alien world. A voice-over tells us that the year is 1973 (voice at the beginning of the film says that it's six months after the initial crash, which was listed as January, 1973) and this is the planet Mars. It transpires that this vessel has been sent to rescue the crew of a previous exploration mission. They have found only one survivor, Col. Edward Carruthers, and suspect him of having murdered the others to save rations for himself. Carruthers pleads his innocence, blaming the deaths of his colleagues on an unknown creature they encountered on the planet.
The commander is unsympathetic and orders the ship to return to Earth. However, before blasting off, a junior crew-member unwisely leaves a door to the spaceship open for a time...
After lift-off, the crew settle down for the long trip back to Earth. It is not long before things start to go amiss: In (now) typical horror-movie fashion, unimportant crew-members wander off to isolated parts of the ship and are dispatched by It. Usually, we see only a character's reaction shot and, perhaps, a looming shadow - the creature, at this point, is not clearly seen.
As the trip progresses, the crew are at first skeptical that something is aboard, but soon have to accept the fact as the body-count mounts. At this point they decide to tool-up - the ship is equipped with an impressive amount of weaponry, including handguns, machine-guns, hand-grenades and even a bazooka.
The intruder is largely immune to all this hardware however, and at one point the crew manage to trap It in the "reactor room" (the ship is nuclear-powered) and expose it to the reactor by raising a shutter (apparently the nuclear pile is like the furnace in a steam-ship).
As the crew dwindle, they retreat up the ship until finally they are in control only of the top-most chamber. In a final stand-off, in which all manner of anti-armour weapons are unleashed in a confined space to no great effect, they hit on the excellent idea of blowing the hatch. The explosive decompression does the rest and the creature is no more.