Sand, in the second half of season 4, is the second Blakes 7 story by Tanith Lee, who also wrote Sarcophagus.
It starts on the sandy planet, where there is lots of sand and a manny saying a poem. He is Don Keller. On a spaceship Servalan is watching a video of Don Keller saying his poem. She is going to the sandy planet to look for him, with three other mannys with her.
One of them is Investigator Reeve, an important Federation manny. Orac has heard about the Investigator going to the sandy planet, so Avon wants to go there as well to get whatever the Federation is after first. Tarrant and Dayna teleport down from Scorpio!
Servalan is on the planet when one of her mannys gets killed by an unseen monster. Reeve thinks it was Servalan who killed her manny. He has recognised her as being Servalan even though she is in disguise as Commissioner Sleer.
Reeve shoots Dayna and injures her so Tarrant gets her teleported back to Scorpio! Some sand goes with her. Reeve goes looking for Tarrant but before he can shoot him he gets distracted by something so Tarrant kills him first.
Scorpio! loses contact with Tarrant, then Orac goes mad and says he loves Soolin so they have to switch him off. They can't teleport Tarrant back up so he is stuck on the sandy planet with Servalan.
Tarrant and Servalan make a truce and go into Don Keller's base together. The computer in the base is mad like Orac. They find Don Keller and he falls down dead when Servalan hits him even though he has been dead for five years.
The base gets completely covered by sand. Tarrant and Servalan are stuck inside together so they get romantic. As both a writer and connoisseur of slash and erotic fanfiction, this seems to me like perfectly normal behaviour from them both.
Here is some dialogue from this important scene:
Servalan: "You are both useful and decorative."
Tarrant: "Thank you. I might say the same about you. But then I'd also say you're possibly the most unscrupulously venomous woman in the galaxy. Being shut in here with you is rather like being locked in a cage with a panther - a black cat with large golden eyes and long silver talons."
Servalan: "Oh Tarrant, I'm just the girl next door."
Tarrant: "If you were the girl next door, I'd move."
Servalan: "Where would you move to, Tarrant?"
Tarrant: "Next door."
I particularly like the bit about a cat. Purr purr.
Meanwhile on Scorpio! Avon is trying to work out what is going on, but Vila is drunk.
Tarrant has already worked out the sand is behind everything. The sand is really a monster that can move about by itself and kill mannys that it doesn't like - which means any manny that it doesn't want to be in its fanfiction. So Avon and Tarrant are safe, but Vila is expendable.
Servalan is sad because she wanted to be paired with Don Keller instead of Tarrant, but he is already dead by the sand. She cries and her tears kill the sand. Servalan kiffs Tarrant and then steals his gun.
This is really the same trick that Avon pulled on the alien in Sarcophagus. Silly Tarrant.
On Scorpio! they find out by accident that water kills the sand. Avon wants to make it rain to kill all the sand on the sandy planet, so they do.
Soolin: "You gambled, Avon, but you won."
Avon: "Do I get a prize?"
They teleport Tarrant back up and he tells them he saw Servalan. Servalan goes back to her ship and flies away, ready to be the baddy again some other day. That is the end of the episode.
This is another clever story for Blakes 7. The sand monster is mysterious at first because nobody thinks that sand could be a monster, until Tarrant figures it out. But the really clever bit is getting Tarrant and Servalan to kiff each other even though they are enemies.
This is just like the best kind of fanfiction, only it is on TV instead of on the internets, and it is Tarrant kiffing Servalan instead of Avon, which would have been much better.
Sadly there is no TV story in which Avon kiffs Tarrant, or that would be the best episode evar. Oh well, a cat can dream... And then write stories on the internets!