31 Monsters of October, Day 21: It (the Terror from Beyond Space)

Real World Origin:
Film, It! The Terror from Beyond Space (1958)

In-Universe Description:
A manned mission to Mars ends in tragedy when the spaceship crashes on the surface. Six months after the crash a rescue ship arrives. The sole survivor, Colonel Carruthers, is accused of murdering his fellow crewmates to preserve the ship's rations for himself. None of the rescuers believes Carruthers' claim that an unseen creature on Mars was responsible for the killings.

It is soon realized that Carruthers was telling the truth when the monster that stowed aboard the rescue ship starts killing off the crew. The creature tends to hide within the ship's air ducts, capturing crew members and storing them in ventilation shafts. Rather than killing its prey immediately, the monster prefers to gradually feed off a victim's moisture and bone marrow.

The crew make several attempts to kill the creature, but find that guns, explosives, and radiation are ineffective. However, one trapped crewman is able to hold the monster at bay with a welding torch. The remainder of the survivors try to put as many pressure bulkheads between them and the creature, moving farther and farther up the length of the rocket until they reach the uppermost compartment.

Warning: Spoilers ahead!

With the creature smashing through the last bulkhead, the crew devise a desperate strategy and don their spacesuits. As the monster crawls through the breach in the door, the survivors open a hatch leading to space and allow the ship's atmosphere to leak out. The monster suffocates within mere feet of the survivors.

This film is often cited as an uncredited inspiration for Alien. Both feature a nearly indestructible alien gradually killing off a ship's crew. Both aliens initially travel through the ship's ventilation ducting and both stow barely living crewmembers away for fiendish purposes (although this scene was cut in the initial release of Alien). Both monsters seem to be susceptible to heat; i.e., a welding torch in It! and flame throwers in Alien. And finally, both creatures are defeated by spacesuited humans who vent the ship's air out an exterior hatch.