I like the way that petty power struggles on Earth and Metebelis 3 begin to pale with the introduction of two awesome, opposing forces: the as-yet-unseen Great One, and K’anpo, the wise Yoda figure back on Earth. The Great One’s minions plot against each other. Lupton, brought before the Parliament of Spiders, proves he’s a very slippery customer as he begins to turn the Eight Legs against Queen Huath. No wonder he was salesman of the year. In response, Huath has a woman-to-woman chat with Sarah Jane and brings her into a counter-plot to shore up the Queen’s own shaky position. That a power struggle between twitching spider puppets should be quite this fun is quite baffling.

But all this is a sideshow to the Great One’s true purpose. She seeks the one perfect crystal that the Doctor found (or stole) from Metebelis 3 centuries ago, and she commands the Doctor to fetch it for her like a good boy. Seeing the Doctor reduced to her puppet, marching round as she laughs at him in that mad, singsong voice, is oddly disturbing, mainly because Pertwee plays it as genuine distress and fear. If the spiders weren’t scary enough, there’s something extremely disquieting about seeing our stalwart Time Lord hero just a ‘little man’ pitted against the deadly power in the cave of crystals.

Probably conscious of this, and with a typical sense of responsibility to the younger audience, Barry Letts introduces several quite funny touches. Sabor gives every impression of being mildly inconvenienced by his imminent consumption by spiders, rolling his eyes and tutting at every tiresome interruption on his route to the dinner table. The Queen/Sarah Jane (this era’s Doctor/Donna) is also fun, blithely teleporting herself and the Doctor out of danger and dangling the TARDIS key at him as they head back to Earth.

There, they meet K’anpo, who thus far has been an offscreen presence like the Great One. He seems to be more than just an old man, apparently recognising the Doctor (‘We have no need for symbols, you and I’), and having a disarming ability to make him fess up to his flaws. His calm serenity, in contrast to the Great One’s hysterical insanity, provides the Doctor with an ally who may be equal and opposite to the force that promises to destroy him.

Next episode: Planet of the Spiders – Part Six