By going from Marco Polo to The Keys of Marinus we’ve taken a trip from the sublime to the ridiculous. That’s not to say that Marinus is bad – it’s good, pulpy fun – but when watching the series sequentially it’s a little jarring to have this sudden shift in tone. But that’s one of Doctor Who‘s great strengths – the fact that every new adventure might play out in a totally different way from the previous one.

You have to admire the ambition of Verity Lambert’s time as producer. It seems that no story was too ambitious to mount (think of the expertly created props in Planet of Giants or the sheer weirdness of The Web Planet) and Marinus is another example of this. With such a tiny budget the notion of creating a new environment in episodes two, three, four and five obviously placed a huge strain on the meagre resources of Ray Cusick. He manages to pull it off quite successfully, although there are times when you do need to be a little forgiving.

The miniatures that open the story are excellent though. The shot of the island – with a model TARDIS (light flashing) then appearing – is a lovely one. The sight of the Voord’s submersibles traversing the sea of acid is less effective though – mainly because it’s painfully obvious they’re being pulled along by wires (and one of them is reluctant to move, so requires a few hard tugs to enable it to reach the beach!)

The sight of the Doctor, Susan, Ian and Barbara surveying this strange new planet shows how far we’ve come since The Daleks. Back then, Ian and Barbara were wracked with fear and doubt (hating the fact that they’d been uprooted from their safe, 20th century existence) but now they regard this bizarre island with nothing more than mild curiosity.

Hartnell’s a bit stumbly over his lines in the early part of the episode (which gives us one of his classic Billyfluffs – “yes, and if you’d had your shoes on, my boy, you could have lent her hers”). This part of the story – as the four examine the beach – is a little problematic. Given that the studio was so small, the beach set couldn’t be particularly large either – which becomes painfully obvious when everybody has to walk around rather slowly.

The moments when they notice the Voord’s submersibles and Arbitan’s building both seem false – there’s no way to imply that they’ve travelled any distance from the TARDIS, so these things must have been under their noses all the time.

I know that examining logical loopholes in a Terry Nation script is a little futile, but the sight of the Voord suit – which contained a man who’s been destroyed by acid – has always irked me. If the suit had a rip then that would have let the acid in, but he was inside a submersible – so that too, must have had a hole (which is rather unlucky really). And if that was the case, wouldn’t the submersible have been full of acid?

Why does Arbitan’s building allow people to enter? You’d have thought it would have made much more sense to keep them outside (and since the Voord don’t seem to have any particular weapons, an impenetrable wall seems as good a barrier as any). Instead, its obvious that although there’s interior defences, sheer force of numbers will allow the Voord to succeed once they do get inside.

It was a bit of a coup to get George Coulouris to appear as Arbitan, although he was no stranger to low-budget British science fiction, having appeared in ITV’s Pathfinder trilogy. He’s the Keeper of the Conscience of Marinus and he explains to the Doctor and his friends exactly what the machine does. “At first, this machine was simply a judge and jury that was never wrong or unfair. And then we added to it, improved on it, made it more and more sophisticated so that finally it became possible to radiate its power and influence the minds of men throughout the planet. They no longer had to decide what was wrong or right. The machine decided for them”.

And Arbitan wants the Doctor’s help to restore the machine’s power! The notion of anybody being denied free will would later become something the Doctor would fight against time and time again (for example, The Masque of Mandragora) so it’s astonishing that he sees nothing wrong with this machine.

Thanks to a handy bit of blackmail with the TARDIS (the Doctor’s still the type of person not prepared to launch into a dangerous adventure just for the fun of it – that will come a little later) Arbitan persuades the four time-travellers to set off for a jaunt around Marinus to recover the four keys that will restore the machine to its former glory.