‘They’re only people.’ The reveal of the Tharils’ true nature – after we’ve already been encouraged to sympathise with their plight through Rorvik’s treatment of Biroc and Lazlo, the imagery of Tharils packed below decks like slaves, and offhand references to ‘Tharil hunts’ – is a brilliant, rug-pulling moment. Biroc sits in his great hall declaring, ‘we are kings’ as the Doctor grimly guesses that they are the enslavers the Gundan Robots were built to defeat. Two time zones – the fall of the Tharil Empire and its ancient ruins – converge, the blade of a Gundan axe slices into the same axe embedded into a cobwebbed table. The show hasn’t bettered the poetry and visual storytelling in this sequence. It’s astonishing.

What’s impressive is how relatively straightforward the rest of the episode is. On the slave ship, Romana bonds with the horribly burned Lazlo, eventually (and with Adric’s help) escaping through the mirrors which heal Lazlo’s wounds just as they’ve healed the Doctor’s injured hand (and hold out hope for K9’s damaged brain). Lalla Ward gets her best material this season; like Emma Peel, even when she’s tied up at the mercy of the villains she’s fearless and commanding. As she sneaks round, she discovers the slavers’ plans to use the MZ and potentially even a “back blast” against the mirrors that have trapped Rorvik’s crew in a void. Meanwhile, the Doctor has followed Biroc (who makes even less eye contact than Tom Baker) through the gateway into a whole universe, memorably represented by black and white photographs.

I like that the script makes sure major plot points are repeated to help the audience: both K9 and Lane point out that the void seems to be collapsing. The mirror’s healing effects are shown twice. This is a more complex story than some, but it’s told in a relatively accessible way. The visuals are imaginative – as in the first episode – but help to tell the story: the slaver ship, the gateway and the TARDIS all seem much closer together in the impressive model shots; the same location in two time zones is easy to understand thanks to the editing. The music is great, too: Peter Howell’s jaunty electronic medieval banquet music is a highlight.

Next episode: Warriors’ Gate – Part Four