I suppose for an ocean-oriented show, this is another cliched plotline, but they did it very well.
Ex-Premier Brynov manages to escape a firing squad and plans a retreat to the United States, where he will plot his comeback at the expense of a war--but only a small war. He's surrounded by his brother, his doctor, his former military head (I think) and one who apparently looks after his finances. I found it interesting that all of them (with the possible exception of Semenev) all address Brynov by his first name, Alexi. Just all buddies together. It becomes clear that Mikhil, Brynov's brother, no longer believes in him as a leader and is prepared to stop him any way he can. As per usual on a television show, Mikhil makes the classic mistake of boasting of his intentions before carrying them out, and Brynov of course is able to retrieve his precious microfilms before they can be tossed overboard.
Nelson comes on board Brynov's ship to verify the value of the microfilms, while Seaview is sent off to a rendezvous point once they've gotten out of the territorial waters. Seaview is attacked on its way, making it clear that Brynov's country knows what is going on, but Brynov is complacently certain that no one will be able to find HIM, even when Nelson hears an approaching plane. A nasty possibility dawns on Brynov--just before the ship is hit.
They pull a sneaky little trick as the scene returns after the break. We see someone who looks a lot like Nelson slip under the water, before the camera pans away and we see Nelson clinging to some wreckage. Nelson finds Mikhil, injured. Brynov's other cronies are in a raft, and they pull the two aboard. Josip makes it rather clear that they do so not from humanitarian reasons, but because they want the skilled Admiral to look after them in the raft. Mikhil dies before he can say anything significant to Nelson, although his incoherent babbling make it plain that Mikhil considered his brother a dangerous man.
Nelson takes firm hold of the situation, and the others obey him unquestioningly--until they locate Brynov, still alive. Brynov seems very amused by the situation, even by his deduction that the traitor must be one of the men on the raft, but you can sense his implacable viciousness under the humor.
Meanwhile, back on Seaview, they have learned of the ship's sinking. There is a brief but intense confrontation between Chip Morton, who wants to go back and search for survivors, and Crane, who insists on following the Admiral's orders and proceeding to the rendezvous point, even knowing that the Admiral may never get there.
There is an ugly scene when the raft is attacked by sharks, and three crewmen who had also found the raft are deliberately knocked overboard by Brynov. Possibly because of 1960's sensibilities, there are no unpleasant sound effects--the men simply disappear under the water. Nelson is outraged, but Brynov is serene and full of self-justification.
Semenev attempts to sell information to Nelson, the price being a drink of water, but Brynov shoots him before he can say anything. Nelson instantly knows that his own death warrant has been signed, but Brynov has no intention of letting him die while his sailing skills are still needed. Dr. Konstantin proves to be the traitor. Josip, whom no one noticed inexplicably "slept" through the sound of two shots and some rather loud talk, grapples with him and they both go over the side. Brynov shows no remorse whatsoever for the loss of his one trustworthy companion.
Nelson does something rather silly, but I can see why the plot called for it. While Brynov sleeps, he takes a drink--and then dumps the rest of the water overboard. The sensible thing to do would have been to drink all of the water, but of course that would have made him look greedy to the viewers.
Brynov has been blatantly menacing throughout the show, but Nelson is rather menacing himself--and far more subtle. There is only the slightest smile on his face as he watches Brynov gulp down some seawater. The smile returns some time later when Brynov wakes up. His thirst has of course been exacerbated by the salt, and he frantically drinks some more, then collapses. Nelson finally gets the gun away from him, briefly contemplates killing him then and there, then briskly makes a radical change in the raft's steering.
The next and final scene is in Sickbay, where Nelson finds that the indomitable survivor Brynov did not make it to the rendezvous point. This scene was very well done. I suppose that they had the two actors so close together because of the camera close-up, but seeing Captain Crane just inches away from his friend, watching him with such concern, was very moving.