The Plot: The space vessel Nostromo receives a distress signal from an unknown planet. After searching for survivors, the crew head home, not knowing that they have picked up a sinister bio-form in the process. The Subtext: In simple terms: rape. A sexual attack on the viewer, Alien is designed to address one of life's more heinous acts, going one further by allegorically referring to the idea of male-rape, a taboo touched upon significantly less than the traditional male-on-female version of the act. Intentionally designed to go after Man's fear of vulnerability, Alien - drawing from the sexually explicit art of H.R. Gieger, who was recruited to design the titular Alien after screenwriter Dan O'Bannon had worked with him on a doomed early adaptation of Dune - purposely ignores going after the women of the picture, instead barraging the men with the allusions and imagery which O'Bannon hoped would "make the men in the audience cross their legs." The allegory is most obvious in the fate of John Hurt's character (Kane), who famously gets raped, pretty literally, by an alien which attaches itself to his face, forcing itself down his throat where it can't leave unless it secretes - if this doesn't happen, the victim will die. After that comes the film's most famous moment, when the alien's spawn - the forced pregnancy, if you will - bursts through Kane's stomach to signify Man's perceived ignorance to the pain of childbirth. The spawn also looks like a penis, just in case the subtext wasn't becoming obvious enough.
