A psych patient named Paul (David Krumholtz, now of Numbers) is waiting to be seen on a busy day in the ER. Carter (Noah Wyle) and Lucy (Kellie Martin) are both occupied with other tasks, including preparations for a staff Valentine's Day party. It's always a bad sign when someone in the ER doesn't have enough time to deal with a patient.
Paul is allowed to roam the halls, and he makes a detour into the staff lounge to pick up a knife. The episode ends with Carter coming into a darkened room, where he's surprised from behgind and stabbed by Paul. After he falls to the floor, his eyes widen in horror because he (and we) see that Lucy has been stabbed as well. (She dies in the next episode)
The first time I saw this episode I literally came off my couch in fright, thanks I think to an effective bit of staging by director/series costar Laura Innes. The stabbing scene owes something to Halloween, with Paul emerging out of the darkness. Seeing the ep again reminded me of watching the first season of ER every week with college buddies, and I guess I owe nutty Michael Crichton some thanks for all the good times.