This ensemble episode begins as a mystery before turning into the old "runaway train with a monster on board" story. (Okay, I admit, most train stories don't do the monster part). Most of the episode takes place on the Defiant, which stands in for the locomotive, and the monster aspect is an extension the season's opening two parter. It's an intelligent but suspenseful story that moves swiftly and challenges the viewer while still featuring plenty of action The Defiant itself includes a gorgeous new engineering set, and the ship's inclusion gives the writers a chance to expand on the character of Lt. Commander Eddington (Kenneth Marshall) while keeping the viewer on his guard as the episode develops into an effective paranoid thriller.
With an ending that vaguely but successfully sets the stage for the future of the series, "The Adversary" is a unique season finale that serves up enough of a conclusion to satisfy the viewer but also has enough of an abstract cliffhanger to entice that viewer to return for the next season.