This episode, season 3's finale, uses a classic plot device that plays on one of society's deepest fears -- that one of us, a completely ordinary-looking individual, is in fact a dangerous outsider bent on sowing death and destruction. In this case, the Defiant's crew is infiltrated by a Changeling who has sabotaged the ship and is trying to instigate war between the Federation and one of its neighbors.
The actors do a really good job of conveying the extreme paranoia of the situation. At a certain point they become convinced (along with much of the audience) that Commander Eddington, a not-particularly-sympathetic character, is the Changeling. Yet the writers pull a big plot twist. The scene near the end with O'Brien and the two Odos is fantastic.
All in all, a wild ride that will keep you on the edge of your seat. Odo's ominous warning to Sisko at the end of the episode is a nice setup for the 4th season.