It’s fair to say that the whodunit mystery in this episode and the previous one is hardly of an Agatha Christie level. Barbara, Altos and Sabetha decide to speak to Aydan’s wife Kala to see if she knows anything. But it it seems not. “Leave me alone. I do understand and I sympathise with you. You must have been sick with worry since you spoke to Susan, but I just can’t help you. I know nothing.”

Of course, she couldn’t have known that Susan and Barbara had spoken (on the space-phone, after Susan was kidnapped) so it proves she was involved – and as it turns out was the one who murdered Aydan. Doh!

Kala’s accomplice is unmasked (not the greatest shock in the world) and the Doctor and the others are then free to return to Arbitan. But none of them know that he’s dead – murdered by Yartek (Stephen Dartnell). Or to give him his full name – Yartek, Leader of the Alien Voord. If my memory serves me right this phrase first pops up in The Making of Doctor Who by Terrance Dicks and Malcolm Hulke, but wherever it first surfaced it’s remained pretty much the only way to refer to him for decades. Or if you prefer the shorterned version – YLOTAV.

Yartek isn’t terribly convincing when he pretends to be Arbitan, but it still seems to good enough to fool Ian, who hands over the final key to him. When he and Susan leave the Conscience Machine he still seems to be putting the pieces together – but afterwards he tells the Doctor that he only handed over a fake key (so was he pretending he didn’t know what was going on to Susan?)

Arbitan wanted all the keys so he could restart the machine and control the Voord. But the Voord had proved to be immune to the machine’s control, so how would that work? But since Yartek planned to use the machine, it seemed that he knew that he and the other Voord wouldn’t fall under its control. Somehow. Possibly after six episodes, Terry Nation had begun to find his attention wandering a little.

What’s slightly irksome is that after they’ve spent weeks searching for the keys, the machine is destroyed – so they might as well not have bothered in the first place. The Doctor then says “I don’t believe that man was made to be controlled by machines. Machines can make laws, but they cannot preserve justice. Only human beings can do that.” A pity he didn’t say that in episode one, it would have saved them a fair amount of trouble!

The Keys of Marinus might be a low-brow romp, but it’s never less than thoroughly entertaining. However, as we prepare to enter the temple of evil, the tone of the series is set to change again.