OK, so, I may have been a bit wrong about the previous episode.
I thought Jo and the Doctor discovered the two maintenance guys on the sea fortress both dead. But no! It turns out only one of them is dead, and the noise the pair heard was the second guy running out and screaming something about some sea devils. That guy doesn’t stay conscious very long, so the Doctor and Jo take him to go lie down for a while and call for help.
Problem: the radio has been damaged, making it impossible to call out for help.
Solution: the Doctor crafts a makeshift transmitter from a simple transistor radio that breaks down in a small explosion shortly after the Doctor gets someone to come get them.
And it turns out the Doctor also sees the weird-headed “sea devil” running around, and while the thing doesn’t stay close for long, the Doctor does as he always does and attempts to communicate with it. Once back at the naval base, the Doctor theorizes to Jo that it was another species type of the Silurians, and he’s even nice enough to explain to Jo what those were since she wasn’t his companion back then. It also means that, once again, the Doctor will try to broker peace between the human race and the Silurians, but there may be a problem there.
That would be the Master, working with the warden of his prison, and he is up to something. Part of that involves walking around the naval base, disguised as an officer, gathering things he needs for the plan he’s planning. Jo spots him from the base commander’s office while the warden had been trying to keep the group busy while the Master did what he was doing.
That means that while Jo goes for help, the Doctor will have to confront the Master himself back at the prison. And the Master says to his warden partner that he will be more than happy to see the Doctor and convince him to play along with the plan.
The Master’s convincing apparently involves a gun.
But this is the Third Doctor. He can karate chop his way out of there, then there’s a sword fight, and just as the Doctor seems to have prevailed, the Master throws a knife and…roll credits.
Man, that Master sure does know how to cheat.