Behind-The-Scenes Pix #35: STAR TREK

Time-traveling back to the late Sixties, we find William Shatner and Leonard Nimoy in the U.S.S. Enterprise briefing room during the shooting of... which classic Star Trek episode? I'm not certain, although the slate tells us it's one directed by Marc Daniels. My knowledge of the series is just slightly shy of encyclopedic, so I can't nail it down from the picture alone.... anyone want to field a guess?