Essentially, this episode takes place after the Natalie/Jeremy breakup. Jeremy finds himself excluded from the group, and goes to a local bar, where he meets a girl. The rest of the gang plays the titular game, in which one person acts like a celebrity and the others try to figure out who the celebrity is. Jeremy eventually learns that the girl is a porn star, which winds up ending his night. Meanwhile, Casey has to break it to Dan that he (Casey) made a prestigious list while Dan didn't.
Ultimately, this is an episode centered on conflict. The Dan/Casey rivalry, usually relegated to the subtext, gets a major twist here that lasts the rest of the series. Sorkin's evocation of the jealousy that comes between friends is right-on. Jeremy loses (for the moment) the girl in which he was very interested, as well as his longtime pals. The humor of the Celebrities game ultimately becomes a sort of shield for the pathos-ridden angst in the Sports Night studio.
However, just these elements don't constitute what makes this episode a standout. It is, ultimately, the finale where Jeremy sits on the couch in Dan and Casey's office and answers all the game's questions to himself that makes it unforgettable. Extra kudos to Josh Malina, for perfectly showing the quiet alienation he feels. It is possibly his best acting moment on the series, and it's hauntingly poignant.
Anyway, that's what I've got to say.