When I first saw this episode in 1993 as part of the 30th anniversary repeats it was in B&W. Now, thanks to a mix of computer colourisation and the chroma-dot colour recovery process, all those greens and purples live again. The Pertwee years restorations, combining B&W films, American low-quality domestic and NTSC broadcast tapes, and “technology worthy of the Doctor himself” often developed by fans of the show, is a story every bit as fascinating as anything to do with Doctor Who, and the availability of every Pertwee episode in colour is possibly the greatest achievement of the DVD range.

Whether this episode justifies the effort and care spent restoring it is another matter. On balance, this has some of the most memorable moments from Terrance Dicks’ 1976 novelisation (one of my favourites), including the cover image plus the beginning of the Doctor’s escape from the Dalek city up the ultimate ventilator shaft. David Maloney’s direction almost favours monochrome, with shadows on the walls as the Daleks pursue the Doctor and Codal through the (film) corridors of their underground base, and the Thals scurry through the wintry warren of icecano tunnels. It feels almost appropriate that an episode that seems designed like an homage to the original Dalek serial, with the Daleks gliding through the archways of their city should be the one to survive only in B&W.

But then, we’d be denied the glorious orange ventilator shaft, a visual signifier of the heat that’s going to lift the Doctor’s cobbled-together hot-air balloon to Spiridon’s surface, and the gorgeous shot of the purple-fur-clad Spiridons shuffling through the Aztec-looking entrance to the Dalek base.

We’d also be denied the Daleks being done pretty much right for the first time since 1967. Getting Roy Skelton back for the first time since The Evil of the Daleks helps in getting the voices right (or at least better): they have an element of unhinged paranoia missing from the robotic versions in Day of the Daleks and Frontier in Space. And the Daleks themselves are a bit more lively and twitchy – or at least the main props are. The new “goon” versions are static, giving the impression of numbers but otherwise being as lifeless as the photo blow ups they used back in the 1960s. Again, the story lacks innovation, but it’s ticking all the right boxes for a celebratory 10th anniversary Dalek story.

Next episode: Planet of the Daleks – Episode Four