The Doctor, Harry and Sarah begin the story in space, then they appear on Nerva as last seen in The Ark in Space. The plot gets going when they find a dead manny, and Harry says he has been dead for two weeks, so clearly something is rotten in the state of Nerva as they find more dead mannys and serious music plays to let us know it is serious.
There are some mannys on Nerva who are alive though, including Harry Mailer from The Mind of Evil and Dev Tarrant from The Way Back, so they are obviously baddys. Dev Tarrant is called Kellman in this and he talks with the other mannys so we can get the exposition required to know what is going on. Nerva is a beacon, there in space to warn other spaceships about an asteroid so they can avoid having a "space collision" with it. (Alas, this flash of Nationesque dialogue is not typical of the episode as a whole.)
An alien in a cave is trying to contact Nerva, but he is shot by two other aliens. There is no immediate clarification of the significance of this so it is very mysterious. The manny on Nerva who hears the signal tells Kellman he thinks it came from the asteroid, which Kellman calls "Voga".
The Doctor opens a locked door with a little help from Harry and Sarah. While there is little humour in this episode, the Doctor's line here is funny:
"If you two would put your weight against the door and stop it opening too suddenly - I don't want to lose my arm, I'm rather attached to it. It's so handy."
Two of the aliens are Vorus, who is played by David Collings from Dark Towers (and lots of other things as well, including Monkey, Sapphire & Steel and Lord of the Rings), and Magrik, played by Michael Wisher who was in only the previous story, but he looks different because then he was Davros and now he is Magrik. Magrik is scared of cybermannys but Vorus is brave, which is a start towards giving them distinguishing characterisation. Together they have some sort of plan but we don't know what it is yet.
The Doctor, Harry and Sarah are captured by Nerva's Commander and Harry Mailer. Kellman wants them to be shot but the Doctor convinces the Commander that he and Harry are doctors and can help with the "plague" that Kellman was trying to blame them for.
Kellman has some of John Drake's old equipment hidden in his room and he uses it to spy on the Doctor and the Commander. When the Doctor hears the nearby asteroid is called Voga he connects it with the cybermannys and knows that the plague is really cybermanny poison.
Kellman uses a radio disguised as a typewriter to send a signal to the cybermanny's spaceship, making him look even more of a baddy than being played by the same actor that played Dev Tarrant.
The Doctor looks in Kellman's room and finds his spying stuff really easily. Kellman comes back and is suspicious so he sets a trap to electric the Doctor. This isn't the cliffhanger though - that is when a cybermouse pounces on Sarah to try to poison her.
This is a serviceable first episode - quite a lot happens, including a parallel subplot with the aliens that remains separate from the plot with the Doctor on Nerva throughout, and the cybermannys only make a brief appearance - enough to make their presence felt without undermining their menace; enough to whet the viewer's appetite for more.
On the other paw, it puts the pieces of the story in place without the kind of wit or panache that it needs to make the episode worthwhile in its own right and, especially for a story following on from Genesis of the Daleks (one of the all time great Terry Nation classic stories), that isn't good enough.