Set in 1973, It! opens with Colonel Edward Carruthers (Marshall Thompson) facing court-martial changes for murdering the members of his Mars expedition. A second spaceship, commanded by Colonel James Van Heusen (Kim Spalding), journeys to the red planet to take Carruthers into custody. Carruthers staunchly maintains his innocence, claiming that his fellow team members were "killed by something--not me."
That something creeps aboard Van Heusen's craft as it blasts off from Mars. After two crew members disappear mysteriously, Carruthers and the others discover the alien stowaway (which even resembles The Thing--with an uglier face courtesy of a fakey rubber mask). The rest of the film concerns the crew's efforts to destroy the creature (played by Ray "Crash" Corrigan). They shoot it with bullets, set booby traps with explosive grenades, hurl gas grenades at it, burn it with a blow torch, and (again recalling The Thing) electrocute it.
It's also unfortunate that director Edward L. Cahn shows glimpses of the creature so early in the film. Screenwriter Jerome Bixby's story opens as a whodunit with no indication of an alien creature's involvement. Therefore, a more intriguing approach would have been for the crew to suspect Carruthers initially when their co-workers began turning up dead. As it is, the film copies The Thing's structure, in which the emphasis is on destroying the monster while suffering as few fatalities as possible.
Despite these limitations, It! The Terror From Beyond Space rates as an above-average, low budget sci fi film. It was Cahn's best effort in the genre, although his follow-up film, 1959's Invisible Invaders, foreshadowed 1968's Night of the Living Dead with its eeries scenes of the dead walking again.
Screenwriter Jerome Bixby's greatest contribution to sci-fi and fantasy was his short story "It's a Good Life." This creepy tale of a young boy who controls an entire town served as the basis for one of The Twilight Zone's most famous episodes (with Billy Mumy as the boy). It was also remade, and significantly altered, as part of Twilight Zone--The Movie.