Oh, it must be "The Guardians," episode 3 of season 2. Yeah, that's the second of four appearances by Carr's Lt. Devlin, and his largest role in the season. He was one of three actors who were used interchangeably in the "supporting officer with dialogue" role, the other two being Alex Hyde-White and Dennis Haysbert. They appeared four times each, with one or two overlaps, since there were only 11 episodes in the season. Haysbert's biggest turn is in "The Hand of the Goral" 3 episodes from the end, and I don't recall Hyde-White ever getting a standout part (even though his father Wilfrid was a regular that season).