When the episode begins, Captain Crane is being tortured and brainwashed (kinda like the beginning of the classic film "The Manchurian Candidate"). An unseen voice (Werner Klemperer's) is ordering him to pledge allegiance to the Peoples Republic and later explains to him his mission....to kill Admiral Nelson!!
Some time later, Crane arrives at the submarine. The Admiral is naturally concerned...as Crane's whereabouts for several days were unknown. Crane gives him a false story about his mother being ill.
During the mission, Crane is uncharacteristically sloppy at his job. He also is short tempered and insolent. It's actually surprising the Admiral puts up with it, though he eventually does what any good commanding officer would do...he backhands Crane!! What's next? Admiral Nelson's death? Or the destruction of the super-weapon aboard the ship that's undergoing testing?
In addition to the story being a bit like "The Manchurian Candidate", it's also quite a bit like the book "You Only Live Twice" by Ian Fleming (the movie bears no similarity to the book at all). All are good stories...so it's not surprising the episode is very good as well. Well made...well worth seeing...and quite exciting.