First broadcast 27 September 1986.

With Drathro (Roger Brierley) about to die, taking the entire underground facility with him, the Doctor (Colin Baker) races to avert catastrophe. In the Doctor’s future, his trial reaches its initial climax.

If the third part of a traditional Doctor Who serial has a tendency to be unrewarding and repetitive, the fourth part has a tendency to be very fast-paced and dramatic. Here there is some properly decent Doctor Who drama – something Colin Baker’s tenure on the series often lacked – as the Doctor tries to talk a sociopathic robot down from suicide to save everybody in both Ravalox colonies. It’s a strong scene, and I feel it is often disregarded by fans because the over-arching trial sequences are so disruptive to the narrative.

It is telling that almost the first thing Robert Holmes’ script down in this episode is write out Katryca (Joan Sims) and the Tribe of the Free. At this stage the characters are simply in the plot’s way. The time turns out to be better spent on Humker (Billy McColl) and Tandrell (Sion Tudor Owen), two young technicians forced to work for Drathro in his laboratory. They have been a constant presence through all four episodes, and actually get some worthwhile resolution here. Robert Holmes was famous for writing amusing little double-acts in his Doctor Who scripts. I feel like Humker and Tandrell get overlooked.

The episode ends with several plot threads deliberately left open, regarding the sleepers from Andromeda, and the nature of Drathro’s original mission. They were not foregrounded quite enough already, and viewers will get disappointed when they find out they are not resolved or even significantly addressed for another nine weeks.

This was Robert Holmes’ final complete serial for Doctor Who; indeed it was his last professional television work before his death later during Season 23’s production. It does not represent anything close to his best work, and one feels that his failing health possibly compromised the quality of his scripts. Then again, the series had a full-time script editor on staff to make sure quality could be maintained. The elements he wrote – the trial scenes – were the source of most of the serial’s biggest problems. The ones that you could point more directly at Holmes – poorly expressed rising stakes, extraneous characters – would likely have been alleviated if the other problems were not there.

As the first serial after an 18-month hiatus, desperately required to re-introduce Doctor Who to a popular audience, it is hard to describe “The Mysterious Planet” as any sort of success. That aside, I have always felt it was a solidly decent storyline produced in a difficult context. For viewers, the Doctor’s trial would continue the following week.