In About Time: Volume 2, Tat Wood and Lawrence Miles ask the question “Did Doctor Who end in 1969?” The premise of their argument is that in 1969 the programme was altered so fundamentally by a wide range of factors (including the arrival of colour, the change to the UNIT format, and the end of the space race) that no episode produced after The War Games is entirely like those before it.

I think an argument can be made that 1976 is a similar watershed moment, and that the impact of various creative decisions made that year explain the way the show developed in the later 1970s, and ultimately triggers a series of events that have ramifications right through to 1989.

The most immediate effect of The War Games is that the Doctor is exiled to Earth for three years, and the kind of stories that had cropped up as an annual event between Seasons Three and Six became the norm, aided by the introduction of a whole military infrastructure to support the Doctor’s earthbound adventures. Clearly, by 1976, that “Yeti on the loo” approach was in decline, but far from defunct – four out of the first 11 Tom Baker stories feature UNIT to some extent, and for a viewer switching off the TV after The Seeds of Doom there’s nothing to suggest that you won’t be getting one or two contemporary Earth-based adventures per year for as long as the show is on the air.

Season 14 seems to confirm this when The Hand of Fear reliably returns the TARDIS to a familiar Twentieth Century science establishment, of the type we’ve seen in some shape or form every season since 1970. But there is a subtle difference – for the first time since The Sea Devils, UNIT aren’t on hand to assist the Doctor. In this respect, The Hand of Fear is a deeply unusual “Yeti on the loo” story. And as with The Sea Devils, removing UNIT from the equation adds more jeopardy for the Doctor – he has no back up. There is no comforting Brigadier on hand to lay on a helicopter and five rounds rapid. Taken in context with Elisabeth Sladen’s departure making the national news, The Hand of Fear has a grimmer tone than, say, The Android Invasion or The Claws of Axos (which is assumed to be set in the same power station). There’s the possibility that Sarah Jane’s burial in an explosion, possession, and wandering into a nuclear reactor could really be curtains for the character. And Professor Watson’s final phone call to his wife is unprecedented – this time, the reactor really might go critical.

After all this, it’s a miracle that Sarah Jane walks out alive (and it’s well known that Hinchcliffe planned she wouldn’t). The audience is in a similar place as we are at Journey’s End, when Donna’s death has been foretold so often that we can’t quite believe she’s made it. But just as in 2008, there is a sting in the tail when the Doctor gets the call he can’t refuse, and forces his most loyal companion out of the TARDIS. Suddenly, all the rules have changed. The Doctor might have been summoned to his death. After all, the last time the Doctor visited his home planet he was “executed” (or whatever the equivalent of losing a life is) and his companions had their minds wiped of all but their first adventures. In that sense, his parting “Don’t you forget me” has sinister overtones – because there’s the very real possibility that if he takes her to Gallifrey with him, Sarah Jane will suffer a similar fate and be robbed of all her memories of this Doctor.

It’s hard to gauge just how jarring this must have been for a contemporary viewer, and how shocking a set up it is for the following story – but in my mind this is Amy dissolving into the Flesh, or Rose Tyler telling us that Next Time we’ll hear the story of her death. The audience is being set up for something awesome.

They got The Deadly Assassin, which is about as big a pay-off as it would be possible to deliver in 1976. After all, this is a series that’s already shown you the genesis of the Daleks. What could possibly top that, other than the Doctor’s first adventure on his home planet and a final battle against his Time Lord arch-enemy? Everything about The Deadly Assassin suggests what we’d nowadays call Event TV. It’s even positioned as the end of a mini-series of adventures, followed by a six-week gap – Steven Moffat must’ve been taking notes.

Creatively, The Deadly Assassin is the climax of Doctor Who as it unfolded in the 1970s. The Doctor begins the decade cast down from Heaven and exiled to one place and time. His return to Gallifrey as its saviour is therefore his ultimate triumph. But it’s also clearly positioned as his ultimate adventure – in the sense that it really feels like it could be his last. Having been stripped of UNIT and even Sarah Jane in the previous story, the Doctor is entirely alone. We’ve never seen him so vulnerable. And he’s going into battle against a Master who is no longer the charming, honey-voiced seducer of the Pertwee years, who has also been stripped to his basics; robbed of his attractive facade, he becomes the very image of Death itself. This, then, is the kind of primal, final battle that Russell T Davies is recreating in The End of Time. However wrong she was on every other count, Mary Whitehouse was right to recognise the Doctor is in extraordinary peril in the final cliffhanger: his body is lying dying on a slab and he’s fighting for his soul in a hellish world of the Master’s creation.

But mention of Mrs Whitehouse reminds us why, behind the scenes, The Deadly Assassin represents a kind of climax as well – because, on the basis of her hysterical complaints about this story, Philip Hinchcliffe was reassigned, and Graham Williams brought in with the explicit instruction to tone the show down. Of course, that doesn’t happen for another three stories – but we also know Hinchcliffe, with nothing left to lose, decided to overspend on The Robots of Death and The Talons of Weng-Chiang which had a knock-on effect on Season 15’s budget and landed his successor with a financial as well as creative crisis.

The legacy of The Deadly Assassin can therefore be felt both onscreen and behind the scenes. Onscreen, after a final, lavish mini-series of adventures, we get the cash-strapped Williams stories which, for all their brilliant attempts to overcome the need to avoid any kind of of visual horror, unarguably look shoddy in comparison to any previous colour stories (and, strikes aside, lose between three and five million viewers). Creatively as well, there is a sudden lack of purpose in the show. After The Hand of Fear, the Doctor only visits contemporary England twice more in the whole of the 1970s. 1978’s The Invasion of Time is a direct sequel to The Deadly Assassin, and completes the work of debunking the Time Lords and making Gallifrey a likely return destination for the Doctor. In the next story, he gets a Time Lady companion and humans – let alone contemporary ones – altogether stop being the main baseline of normality for the show.

In 1970-76 there is a constant tension in the Doctor’s relationship with UNIT and the Time Lords: he craves his freedom but accepts his responsibility for defending Earth and intervening on behalf of his people – even after he regains the use of the TARDIS and regenerates. After 1976, this tension is lost, and the Doctor is again free to wander, more in control of the TARDIS and his adventures than he’s ever been, with no “Establishment” to rail against. In response, Williams introduces the Guardians as the godlike replacements for the Time Lords who can send the Doctor on missions, and then the Randomiser as an abortive attempt to go back to basics and recapture the “TARDIS on the run” feel of the 1960s, to try to find a viable alternative to the “Yeti on the loo” format.

When this all gets too much for Williams, and Nathan-Turner comes in, there’s a new aesthetic, but an ongoing commitment to trying to go back to basics – in that sense, the creative decision to introduce the Randomiser is more important because it’s about trying to make Doctor Who “like it used to be” than stopping the Doctor from steering the TARDIS. With the rise in organised fandom (pretty much united in their condemnation of The Deadly Assassin), that appeal to Doctor Who’s past, and to the creative imperatives of a previous era of TV becomes deeply destabilising. The over-abundance of companions in Season 19 (including one who’s desperate to get back home) is an attempt to go back to the 1960s, even though there is no production need either to split stories between multiple characters to facilitate “as live” scene changes or to take the pressure off the leading man in an age of all-year-round recording. Equally, The Invasion of Time is a precedent for a string of sequels and the wheeling out of monsters that haven’t been seen for years and are presented as a greater treat for the audience than they actually are. Arguably, it isn’t until 1987 that Doctor Who finds the viable long-term alternative to the “Yeti on the loo” format – by which point, no-one’s watching.

So, did Doctor Who end in 1976? Self evidently it did not. It didn’t even become “worse” in the sense that many of the best stories were made in the 1980s. However, I do think the show became less consistent and for a long while floundered about searching for a useful new direction, toying with Guardians, Randomisers, ”Hard SF” and ultimately its own history without ever settling on one for very long, and shedding viewers as it became increasingly self-reverential. Equally, changes behind the scenes caused their own issues: two script editors oversaw Doctor Who in 1970-76. In the following six years there were six.

The Deadly Assassin is a fitting climax to seven years of Doctor Who, but unlike The War Games, it fails to provide a roadmap for the seasons to come, and leaves a legacy from which it takes the show 11 years to recover.