Film 1 is the crew desperately trying to make it home in the prologue and dying, then the alien mind control worms slowly take over a NASA research facility until it's finally eradicated in the closing scene, with Sigourney Weaver the lone human survivor. But if you wait until after the credits, you get a 5-second teaser of a single worm wriggling out through a crack in the side of the facility.

The "fi" being the important part here.