Turlough (Mark Strickson, on debut) hates his school. So far, so much teenage angst. He longs to escape, so he does so – by stealing a vintage car belonging to one of his teachers. It’s an act so brazen that his fellow schoolboys are moved to cheer politely as the car motors away. Paddy Kingsland plays a jaunty tune on his electric music-o-phone in support. It’s very different from the high school I attended in the 1980s, where if any of the students had stolen a teacher’s car, the assembled crown would have roared, ‘ya faaaarken idjut!’ as AC/DC thundered from the stereo.

Or perhaps that’s just an Australian thing. Tegan would get it. She’d probably be one of the loudest shouters.

Turlough crashes the car and wakes up in a garish video effect. There he meets the gravelly voiced Black Guardian (Valentine Dyall) to whom Turlough confesses that he hates Earth and wants to leave. The Black Guardian’s only too ready to offer the boy a Faustian pact: kill the Doctor (Peter Davison) and he’ll get Turlough off the planet. But what’s never explained very clearly is that Turlough is actually from another planet. For now, viewers (which included young Spandrell back in 1983) just have to wonder who this strange young man is and how he knows about transmat capsules, tangential deviations and warp ellipses.

Such vagueness would never happen these days. If Turlough was in Class, his extra terrestrial origins would have been painstakingly explained in Episode 1. By Episode 2 he’d be covered in gore and by Episode 3 he’d be getting laid by an alien.

Meanwhile the Doctor, Nyssa (Sarah Sutton) and Tegan (Janet Fielding) are busy explaining the plot of last week’s story to each other, when the TARDIS nearly collides with another ship. The Doctor manages to land the TARDIS on board the ship, which is decked out like a gaudy leagues club, all fake marble and ruby red walls. The pokies lounge is surely just around the corner.

Our heroes wander round for a sticky beak, impressed by such advanced technology as motion sensor lighting and the spraying of pot-plants gold. Paddy Kingsland sets his music-o-phone to ‘ominous’, but in truth, it’s all a bit sedate. There’s some flummery about the Doctor needing to transmat to Earth to fix some wires which separates our team and leaves Tegan and Nyssa TARDIS bound, the idea being that they’ll follow the Doctor to wherever the capsule lands.

The ship’s transmat capsule, it transpires, shuttles between the spaceship and Turlough’s school, enabling the two plot strands to combine. But when the Doctor gets to Earth, the TARDIS doesn’t follow. It instead transports Nyssa and Tegan back to 1977, which was when the fancy spaceship was last in range of the planet. So we end up with our two sets of heroes in the same place, but in separate time zones. (Honestly, it says something about Mawdryn Undead that it takes half a post to outlinethe plot).

In 1977, Tegan and Nyssa find a terribly injured man in the transmat capsule and assume it’s the Doctor, suffering a post-regenerative mishap. The man says he’s the Doctor but the Doctor’s friends are not sure he’s telling the truth. The ‘is he/is he not the Doctor’ storyline has mileage, although as the action cuts back regularly to the Doctor in 1983, we viewers always know this fellow’s an imposter.

But there’s the start of a more interesting debate as the newcomer’s features become less and less humanoid. By the end Part Two, spaghetti bolognese is bursting from his skull and his eyebrows are starting to reach around his head. Tegan, already suspicious of this would-be Doctor, becomes even less convinced the less he looks like the Doctor she knows. “It wasn’t like this before,” she says.

TEGAN: When he changed, he turned into a human.

MAWDRYN: Is a Gallifreyan human?

TEGAN: He was normal.

MAWDRYN: What do you know, prattling Earth child, of the endless changing?

TEGAN: I know that when the Doctor changed, he didn’t turn into an alien.

She turns out to be right, of course. Mawdryn (David Collings) is not the Doctor, but there’s more than a hint of close-mindedness in Tegan’s assertion. How would she have acted if figure in the capsule had been black, or disabled? What if Mawdryn had been a woman? Could she have imagined a Doctor who looked like that?

We don’t know, but this whole plot strand is based on the idea that it’s plausible Mawdryn could be a post-regenerative Doctor, but only while he fits a generic physical profile. When he begins to differ from that profile, doubt sets in. In Tegan’s response, there’s an quiet reaffirmation that a ‘normal’ Doctor is a white, able bodied man. No wonder some people still have trouble with the idea that the Doctor might one day be played by someone different to that norm.

In 1983, the Doctor meets his old friend Brigadier Lethbridge-Stewart (Nicholas Courtney) who has given up running UNIT and started teaching maths at Turlough’s school. The Brigadier is in a diminished state, having suffered a nervous breakdown. The previous season, one of the Doctor’s companions died. Here, one’s got PTSD. The Doctor’s not so safe to be around these days.

If the Brigadier’s deterioration is any indication of the effect of being part of a public school, you can see why Turlough wants to leave. It seems to have had a stultifying influence on him, setting up in that shabby little hut. Not only has he lost his memories and freaked out, he’s also developed a taste for corporal punishment (‘I trust you flogged him within an inch of his life!”) which seems unlike the old soldier we used to know. When the Doctor asks about why he left the army, there’s the hint of desperate self justification in his responses (“Bit of admin. Bit of rugger… I happen to like teaching”). It’s no wonder by the time we get to Battlefield, he’s turned his back on this career cul de sac.

So the Brigadier is not well, and neither is Mawdryn. As all aspects of this story converge back on Mawdryn’s spacecraft in Part Four, the plot finally begins to become clear. As the Doctor says, Mawdryn’s people are “Fools who tried to turn themselves into Time Lords. It all went disastrously wrong.” To save themselves from living forever in torment, Mawdryn plans to blackmail the Doctor into giving up his remaining regenerations to allow he and his comrades to die.

This story has already been about so many things, that it’s a surprise that in its last leg, it seems to be advocating for assisted dying. And after some backwards and forwardsing, the Doctor does agree to help, though his decision to aid their euthanising is morally mitigated when the two Brigadiers touch and zap. Mawdryn and his buddies get the death they wanted but through a fluke of chance, so everyone’s conscience remains clear.

But let’s not end on such a gloomy note. I want to talk about one other quirk of this story. The plot requires an object to link the worlds of ’77 and ’83. So Nyssa insists that Tegan doesn’t leave the TARDIS without a homing device, which will eventually land up in the hands of the Brigadier.

You can imagine such a device would be handy is on an unfamiliar alien planet, but Tegan’s back on Earth. What’s more, the TARDIS is parked on top of a hill, next to a giant obelisk. You still need a homing device with all those clues, Tegan? Ya faaaarken idjut.

LINK TO Terror of the Zygons: consecutive Brigadier stories, eight years apart.

NEXT TIME: It’s volcano day. We feel the heat of The Fire of Pompeii.