Ep 12. Kansas

The crew travels through a wormhole to Earth, 1985, and Crichton finds he has somehow disastrously changed the course of time - his father Jack is now slated to fly the doomed Challenger mission. Lying low in Crichton's home town, the 'aliens' use the timely Halloween celebrations to remain (almost) anonymous, while Crichton tries to revert the timeline - in the process putting his younger self in mortal danger.