‘Not enough time.’ There’s a line of thought that suggests The Caves of Androzani succeeds because it was written with Tom Baker’s Doctor (or even a generic Doctor) in mind, and that Davison is essentially playing a different character than in the last 65 episodes. I don’t really buy that: everything the fifth Doctor does here feels like a natural endpoint for his character. We’ve seen the single-minded determination in the face of disbelief and suspicion, in Snakedance; we’ve seen him rail against the petty obsessions of villains, and breezy insouciance in the face of mortal danger. Here, with the stakes raised, all those elements are upped – but they aren’t unlike anything in Davison’s previous performance. He even gets to do something useful with the celery, and his costume finally proves its real value as perfect camouflage on the 86% of planets that look like Dorset sandpits.

Carefully planned or not, it also feels like the right end for the fifth Doctor – the Doctor who failed to save Adric and who’s ever since been haunted by that failure. It’s fitting his last word should be ‘Adric’, and it’s fitting that, having just lost three friends in quick succession, he would literally rather die than lose a fourth. You could criticise this for being another Season 21 massacre, but the pay-off to all those stories that ended with everyone dead, and the Doctor looking miserable is a story where the violence is futile and saving one good person is a victory against all odds. It’s definitely more powerful than falling over pulling out a plug.

Peter Davison’s performance is indeed astonishing, more so for happening almost in isolation from the bloodbath going on around him. Christopher Gable is equally astonishing: scuttling around like a bottled spider, howling as Peri cowers from his horribly mutilated face, seeking final solace in the arms of the robot Salateen. The plot threads all tie up beautifully, with Chellak’s determined assault on Jek’s base finally breaking the impasse just at the point when it doesn’t matter anyway. The poisonous toad Morgus has condemned himself – his disbelief of the Doctor’s honest motives has led him to bloody his own hands and left him vulnerable to Timmin’s brilliant power grab. He walks straight into Jek’s lair and his own death. Stotz, laughing as he guns down Jek, is himself killed by the robot Salateen, sliding out of the shadows like Michael Myers in Halloween.

Graeme Harper’s direction – backed by the doomy bells of Roger Limb’s score – is so dynamic you don’t really think about the oddness of Jek not keeping a supply of the antidote to the deadly substance he’s surrounded by, or the Doctor not giving himself and Peri the cure before running back to the TARDIS. Between machine gunning, mudbursts and fire, and the voiceover of Jek intoning, ‘She’s dying Doctor’ these final moments are a crescendo of action that carries through into the regeneration itself. The second half of this episode is better than anything the show’s produced to date: truly remarkable.

‘Change my dear, and it seems on a moment too soon.’ The new Doctor Who explodes like a chest-burster from the corpse of his predecessor, immediately checking to make sure his own spectrox toxaemia is cured. Though he is never less than brilliant, Peter Davison was poorly served by a lot of the material he was given, particularly in his second series. I’m glad between Dicks, Bidmead and Holmes his final year was largely better, and that he goes out on this new high watermark for the show. There’s a parallel universe where he changed his mind in time and stayed for a fourth season. I wonder how that might have looked.

Next episode: The Twin Dilemma