An ore processing barge called the Nostromo recieves an unknown alien message, and the onboard computer wakes the crew to investigate. The crew investigates, finding an abandoned alient spacecraft filled with strange eggs. One of the eggs hatches, attaching a parasite to a crewmember, who is taken back up to the ship. After the parasite falls off, the crewmember appears fine. Alien promptly bursts out during dinner, systematically kills all the crew over a period of time, and is finally defeated by Ripley (Sigourney Weaver).
This is the theatrical trailer without watermarks.
Director's Cut Trailer
Alien Anthology Blu-Ray Trailer
Alien Anthology Blu-Ray Teaser Trailer
This British TV spot for the movie, which unfortunately cuts off at the end, was originally posted to YouTube by philo1978.
TV Spot #2