After the suspense of last week’s Boom, the new Doctor Who season cranked up to full-on horror in 73 Yards, with an episode destined to be remembered as one of the all-time great companion performances.

Once the Doctor vanishes, it is left to Millie Gibson as Ruby Sunday to unravel the mystery that then stretches through the whole of her life, morphing from creeping dread in rural Wales to a political thriller split between London and Cardiff. Aneurin Barnard cut an impressive figure as the paranoid and controlling populist politician Roger ap Gwilliam, somewhat in the mould of Emma Thompson’s Vivienne Rook in Russell T Davies’ Years and Years.

Michelle Greenidge as Ruby’s mum, Carla, was a delight, wittering on about how men go off to “potter in their sheds” and dismissively saying it was only “Welsh people” who were scared of the mysterious woman. But her delivery to a locked-out Ruby of: “Even your real mother didn’t want you” was conveyed with conviction, and abandoned or orphaned children remain a persistent theme of this run of Ncuti Gatwa episodes.

Davies’s knack of writing side characters who manage to convey a lot with very little dialogue was on display with Sophie Ablett, who gave Marti the air of someone haunted like Ruby – though in her case by the attentions of ap Gwilliam.

The highlight of the episode was without doubt the oppressive Welsh pub night-time sequence, which veered wildly between the hysterical laughter of local people enjoying pulling the leg of the gullible English tourist and the genuinely unnerving undercurrent of menace as they talked up tales of Mad Jack stalking the night.

All of this was absolutely brilliant … until the ending. There didn’t seem to be any logic or explanation of how the older Ruby had been able to appear that way to herself for decades, or spring back in time at the crucial moment to prevent the timeline being created, beyond some vague hand-waving at “the supernatural” mentioned along the way. A shame, as it was shaping up to be a stone-cold classic until older Ruby appeared and the final act faltered.

Sum it up in one sentence?

Ruby Sunday saves the world from a Welsh nuclear megalomaniac after a lifetime of running away from herself.

Life onboard the Tardis

This was a “Doctor lite” episode, a format introduced in 2006 to lessen the production schedule for the leads, first with the mostly unloved Love and Monsters, and more successfully a year later with Carey Mulligan leading Blink. In 2008, the leads each got a showcase – David Tennant helming Midnight, and Catherine Tate’s Donna the focus of Turn Left. During that Davies-penned episode, Donna lives through an alternative timeline without the Doctor and ends up saving the day to reset events. Sound familiar?

Fear factor

73 Yards went for an instantly recognisable folk-horror atmosphere with the woman first pursuing Ruby over the clifftops, though the sequence where Ruby saw her every time she looked out the train window took it to a new level. But the scariest thing was the gradual hardening of Jemma Redgrave’s face as, after an initially reassuring sudden arrival in the story to assist Ruby, Kate Lethbridge-Stewart’s heart was hardened when she and her Unit troops heard what the old lady told them.

Mysteries and questions

The obligatory appearance by Susan Twist as the hiker, her sixth different character in the last seven episodes, did finally spark some recognition. But since we were in an alternate timeline, who knows if Ruby will remember her face the next time she crops up?

Deeper into the vortex

• The theme of being cursed and your family and friends turning against you was also explored in the Sarah Jane Adventures episode The Curse of Clyde Langer.

• The music playing when Ruby discovered her purpose was Watch Me by Labi Siffre, a hit in 1972. It includes the lines: “Watch me when I’m on my own / See me falling like the snow.” Snow has been a recurring motif this season, and Old Ruby told her carer she used to be able to make it snow.

• Ruby telling the mysterious figure: “We’ve got work to do” echoed Sylvester McCoy’s closing monologue in the last episode of the 1963-89 run of Doctor Who, which ended with him saying: “Come on, Ace, we’ve got work to do.”

• The episode did not have an opening title sequence for the first time since Sleep No More in 2015, which itself was the first time it had happened during the regular series. It also did not appear to feature anything extraterrestrial or alien except for the Doctor or the Tardis. Well, perhaps, until we find out more about Susan Twist or Anita Dobson’s Mrs Flood character, who had a cameo.

• Roger ap Gwilliam became the fourth UK prime minister whose career came to a Doctor Who-induced end, although at least he escaped with his life. Harriet Jones and Jo Patterson both got exterminated by Daleks, while the Master was shot by his wife and refused to regenerate while posing as PM Harold Saxon.

• The pub’s name in Welsh – Y Pren Marw – translates as “the dead wood”, and the scenes were filmed at the White Cross Inn in Groeswen, which was previously used as a location for Torchwood.

• Gatwa’s clifftop costume has come in for some criticism as being the least Doctor-ish costume the lead role has had for some time, but old habits die hard and the Doctor has a history of beanies-by-the-sea.

Next time: Dot and Bubble

The trailer suggests we might be about to get a hyperreal morality tale about the dangers of social media and cancel culture …