The Doctor’s flashback to monsters of yesteryear, and an increasing willingness to draw on the show’s backstory, recalls the second Doctor’s defence at his trial, and points the way to more clips to come – in Day of the Daleks and The Three Doctors, and then as an annual event during the JNT years. It opens the episode on a high point – the Doctor’s hearts nearly give out under the mental pressure of the Keller Machine and the Master rushes in to save him.

Delgado plays the scene with concern verging on panic: the Master is clearly desperate for the Doctor not to die. Far more so than in Terror of the Autons we get a sense of their strange love/hate relationship. The only two Time Lords on the planet, they have far more in common with each other than anyone else on the Earth. It’s an idea emphasised by the direction, which includes a very effectively done cross fade from the Master’s face to the Doctor’s, and because the episode features both of them under attack by the alien mind parasite – the Doctor fearing the evils he has fought, the Master fearing the Doctor’s mockery.

Delgado’s performance hits a high here: so much is conveyed without dialogue: his desperation to save the Doctor, and his fear and apprehension about the rebellious mind parasite. A key scene of the episode features Delgado, alone, having to act against the fairly unimpressive Keller Machine prop, ending with him fleeing from it, defeated. He sells it absolutely. Elsewhere, Timothy Combe has to rely on all the electronic effects the BBC can muster, warping the picture to show the machine teleporting about the prison, and flooding it with static when the parasite kills a prisoner.

The prison scenes really work, as does the very Season Seven-y film sequence of the Master’s hired goons stealing the Thunderbolt missile. The details are all very well done, which helps to distract from the plot, which feels pilfered from You Only Live Twice. Quite why the Master wants to provoke a nuclear holocaust that will devastate the world he’s stuck on is, at this point, anyone’s guess. But the script doesn’t dwell on the improbabilities, and you only think about the illogic after you’ve enjoyed 25 minutes of Delgado’s acting, Combe’s direction, and a really effective action set piece

Next episode: The Mind of Evil – Episode Five