The opening teaser is quite exciting. It's rather surprising to see Edith Nelson defending herself so vigorously, but this becomes understandable later in the episode.
Nelson has been informed of Miss Nelson's kidnapping and prepares to meet with the kidnappers. In his cabin, he drops her passport on the floor. Crane picks it up and starts to look through it, prompting Nelson to snap at him and snatch it back. It's rather odd that Crane would look through the passport at all; the reasonable assumption would be that it was Nelson's passport, so why look at it?
Nelson's excuse for the whole situation is that his sister has been involved in an accident, which Crane accepts unquestioningly.
Nelson meets with the kidnapper at a sidewalk cafe. (Classic cloak and dagger stuff.) He's escorted to the hideout and searched for the map that he's to exchange for his sister's life. Nelson has it hidden too well, and demands to see his sister alive and well. We get our first view of something that would become standard in the second season: a videophone. Quite a sophisticated one, too--while talking to Miss Nelson, the camera pans back to show her companion in the car--without any visible adjustments done with the machinery!
Miss Nelson makes it safely into the American Consulate after Nelson hands over the microfilmed map. Fenton, the kidnapper, insists on verifying the accuracy of the map, which can only be done on Seaview. He assures Nelson that they will be able to get a man on board--who turns out to be Fenton himself, one of two special security men assigned to the mission. I loved the look on Nelson's face as he heard Fenton's voice behind him--he clearly reacts, yet keeps himself under enough control that Crane doesn't notice anything wrong.
With a little tweaking, they could have made this episode a follow-up to "The Saboteur", which would have been really appropriate. Both deal with an extensive system of underwater missiles placed as a deterrent to war--only in "The Saboteur" they belonged to the United States, and here they are part of the "Western Alliance".
Colonel Hamid, the second security officer, is, as he says, "paid to be suspicious" and looks very carefully into the situation of Nelson's sister. Nelson's story and the facts do not mesh. Hamid points this out to Crane, but Crane, naturally, does not want to think ill of his friend, and takes quite a lot of convincing. Hamid urges him to have some investigating done and have the information sent to him via his private code. Unfortunately, there's not much privacy on board a submarine. Rather than quietly calling Crane to the radioshack, Sparks makes a shipwide announcement that his private message has come in. Easy enough for Nelson to guess what the message was about, and respond accordingly.
Crane is finally convinced enough to shut down most of the ship's workings so that Hamid's listening device can work unhampered--and what they hear makes it seem pretty clear that Nelson is a traitor. Hamid goes to arrest Nelson. The resulting fight is very well done. The lighting is dimmed enough that Basehart's stand-in doesn't look too obvious. Hamid fights very well, as you would expect of a security specialist. At least, he fights well until the end, when he leans across the desk and puts his face right in the path of Nelson's fist.
At this point, for the purposes of plot, Nelson has to leave his cabin so that Fenton can sneak in and murder Hamid. He goes clear down the hall and around the corner to find a microphone and call the Master at Arms. Those microphones are everywhere on the ship, and you would certainly expect Nelson to have one in his own cabin!
Having been slowly, painfully convinced of the Admiral's treason, it's appropriate that it took Nelson some time to persuade Crane otherwise. After seeing Nelson in action during this season, it was obvious that he was working very hard to keep a step ahead of Fenton. Now we find that he's not just a step, but several steps, ahead. Edith Nelson, in reality, is safely hidden away back in the States, and the kidnapped woman was an Intelligence agent impersonating her. The map given to Fenton is a fake--with only the position of the first missile accurate. Fenton has to be allowed to report back to his people so that they will believe that the map is real. Unfortunately, Fenton decides to take along the special missile detector and have it copied. This means that Nelson will have to go with him, as the detector has to have two security people with it at all times. And that means that Nelson and the detector will have to be retrieved after Fenton makes his report.
Nelson suddenly opens a cabinet and pulls out some spygear. The man's well prepared. He has a special spray that, with the help of certain lenses, will leave a clear trail of glowing footprints to follow. (Courtesy of Artemus Gordon of the Secret Service. A good spy gadget is a thing of beauty and a joy forever.)
It's not surprising that Crane mostly chooses to hold the lenses in his hand and put them up to his eyes when necessary, because it looks rather odd for a man to be wearing dark glasses at night. Crane and his team are dressed like merchant seamen--appropriate disguise for a seaport. One of the men is an unknown face. You know he's not coming back. Crane and the others knew it too--after breaking into the hideout and finding Nelson, they escape before the building blows up. Crane calls to Morton and Kowalski--but not to the unknown man, even though he couldn't possibly have known at that point that the man was already dead, shot while giving the Admiral the chance to grab the gadget and run. (And he was shot after the Admiral left the room, so Nelson didn't know, either.)
All in all, it was a fun close to a marvelous season. (And "Miss Nelson" apparently enjoyed working with Nelson enough that she moved over to the Nelson Institute to work with him some more the following season.)