‘You’re the most important woman in the whole of creation.’ Another Doctor-lite episode that becomes a series stand-out. The benefit of Turn Left against Love & Monsters or Blink is that the companion is present throughout, and so this becomes Donna’s own Father’s Day or Human Nature, as she has to hold the line without the Doctor’s help in her own version of The City at the Edge of Forever plot.

This gains a lot of power from coming at the end of the fourth series, at a point when the new series had its own hinterland of stories to pull on. In a way, this is a continuity fest, rewriting everything since The Runaway Bride to remove the Doctor. This means even relatively trivial “romps” like Smith and Jones or Voyage of the Damned suddenly become terrible glimpses of what might have been: the disappearing hospital is returned with one survivor and it’s not Martha. Florence’s threat to wipe out half the Earth has been foiled but at the cost of Sarah Jane and her children’s lives (there’s an ex post facto frisson of horror as Elisabeth Sladen appears in a BBC obituary).

The daftness of a Titanic-shaped spaceship crashing towards London becomes a Threads homage as the city is wiped out by a mushroom cloud (fortunately, Sylvia manages to control her bladder, although the names of everyone who died will keep coming to her at inopportune moments for the rest of her life). Super-size America is taken off the board by the Adipose. China and Europe’s reliance on petrol becomes their downfall as Torchwood sacrifice themselves to stop the Sontaran stratagem.

The references go beyond the new series: the Time Beetle looks like a Metebelis Spider, and is part of the Trickster’s Brigade (acknowledging that Whatever Happened to Sarah Jane? got here first). The circle of mirrors recalls Kinda, and another companion possessed by something nasty. The apocalypse is depicted in much the same way as in Part Four of Logopolis.

The sum of all this is probably the darkest RTD would allow the show to get. The last day of Turn Right Donna’s life sees the dawn of a new Holocaust – an idea so awful the show stumbles away from contemplating it – and leaves Sylvia reduced to a husk, all the fight gone from her. But the final straw is when Wilf is unable to provide any further comfort to Donna as the stars blink out. At that point, even Donna knows the game’s up.

So she turns to her mysterious saviour, the blonde Bad Wolf walking between universes searching for her Doctor (apart from one fun moment when we hear her literally being fired out of the Dimension Cannon). There was much merriment at the time over Billie Piper’s slightly odd delivery, as though she’s trying to get her mouth to remember the shape of Rose’s speech patterns. With distance, this is less distracting (let’s assume the cannon put her teeth on edge), and it’s easier to pick up on everything that’s right – Rose has become increasingly Doctorish since we last saw her, just as Jackie predicted, and her single-mindedly ruthless determination to be reunited with the Doctor is completely in keeping with The Parting of the Ways. The result is an episode full of heartbreak, horror, ominous portents about Donna’s future, and a cliffhanger that’s among the greatest.

Next Time: The Stolen Earth