Pace has slowed to a crawl in this instalment, as it tends to with most fifth of six episodes. A great deal of time is spent battling creatures in the dark (represented by some light bulbs) which reinforces the dangers of Spiridon, but doesn’t do much to move things forward. You can sense how much the serial has run out of steam by the amount of time the Doctor and Jo sit about wondering what to do next, with no particular sense of urgency.

It’s far from being a wash out. There’s a good jump scare when Codal pulls back the cowl of what he thinks is Taron’s stolen Spiridon purple Yeti costume, only to find no face. And the Dalek plague sub-plot, inserted to give a bit of material to the middle of the story, is nasty. There are deeply uncomfortable overtones as the Spiridon slave workers are herded into the Dalek base for ‘treatment’. And the resolution is effectively brutal: Wester sacrifices himself to unleash the plague inside the Dalek laboratory, forcing it to be sealed forever to prevent the contagion infecting the non-immune Dalek invasion force. There’s a nice thrill of horror as one of the Dalek scientists proclaims, ‘We can never leave here!’ that helps suggest the living beings inside the metal shells.

There’s another moment of Dalek panic when the Doctor decides to pull the old “hide inside a Dalek” trick last seen in the Hartnell years. As he and the Thals push the Daleks into the ice swamp, the Daleks plaintively scream, ‘Assist, assist!’ Unluckily for them, the Dalek mutants really feel the cold (perhaps some sort of car heating inside the shells might be in order?) and die immediately: we don’t see the creatures inside, just a demure splash and a look of distaste on Rebec’s face. However, the idea of the thing inside the machine is obvious, and it’s one that the show will keep returning to through the 1970s.

Next episode: Planet of the Daleks – Episode Six