Douglas Adams, who reworked the conclusion to the story, and the season, immediately spots that the Doctor, when presented with ultimate power, must reject it, otherwise he stops being 100% rebel Time Lord and becomes an authority figure. The way this riffs on ideas raised in Part Five (where the Shadow dismissed the Doctor’s interest in the irrelevant side-show of the Atrios/Zeos War), with the Doctor rejecting the idea of sacrificing one life for the universe, is perfect. Having Astra be the sixth segment is a great idea; having the Doctor decide her life is more important than the Guardians’ endless cosmic to-and-fro is sublime.

The rest of this can’t possibly live up to the poetry of that moment, and sure enough, it doesn’t. A lot of it suffers from end-of-season that’ll-do-ness, like the Mutes’ patent leather boots which are more Shoe Zone than Shadow Planet. The whole shrinking sequence feels like playing for time, and the Shadow, at his most Emperor Palpatinish when the Key of Time is in his grasp, horribly exulting in Astra’s sacrifice, ends up getting written out in a very boring way. The Marshal likewise gets a final moment of insanity – ‘Beautiful mushrooms will blossom and burst’ – before vanishing.

Even so, this is much better than its (pretty bad) reputation. I think it’s largely better than The Invasion of Time and less interminable than the shorter Underworld. It’s not as good as anything in the first half of this series, but it gets the important moments right in a relatively surprising way. It’s got some decent jokes (like the Doctor’s plan depending on K9’s acting talents, and the dog’s ostentatious little cough before he delivers his lines to the Shadow), it’s got a nicely stated moral dilemma (‘Her every living cell is part of this Key of Time, and that to save us, Astra must be destroyed’) and it’s got a Doctor and companion forming a pretty convincing double act. Sadly, for the second time in as many seasons, Graham Williams has let the clock run out on properly writing out the co-star. I think it’s a shame Mary Tamm left, because I think as written by Douglas Adams she’s a great companion. But with all this talk of metamorphosis and Destiny, what does Lalla Ward know…?

Next episode: Destiny of the Daleks