★★★★★ Rona Munro, where have you been? It’s 28 years since she wrote for Doctor Who, back when Sylvester McCoy was the Time Lord. Survival, which I praised as “a wonderful set of scripts”, marked her debut and inadvertently brought the original 26-year run to a close.

What’s often rewarding about Doctor Who is that – beyond rewrites, budget constraints, casting and performance – it allows an authorial voice to sing through. It happened for Munro in 1989’s Survival and does so again in The Eaters of Light. It’s a beautifully written script that feels at one with half-remembered Celtic myths.

“Everyone knows there are ghosts in the hill,” warns the boy at the start of the story. “I want to hear the music,” says his sister Judy, before pressing her ear to the grass. It could be a moment from Rona Munro’s own childhood. She hails from Aberdeen. She’s steeped in the mists and peat of these dreich, windswept Caledonian hillsides and has been captivated by local legends since her youth.

Hers is a distinctly Scottish voice – at least, it seems to be, based on the evidence here; although she’s a prolific playwright, I’m not au fait with her other work. This is an element Doctor Who would benefit more from – an odd thing to say when Steven Moffat is showrunner and Peter Capaldi and Michelle Gomez feature week after week. Scots all. Maybe it’s my Scottish ancestry bubbling to the surface. Surprisingly few stories have been set in Scotland (The Highlanders, Terror of the Zygons, Tooth and Claw, Under the Lake). None of them, including The Eaters of Light, was filmed there.

Both Survival and The Eaters of Light concern adolescents running wild without adult guidance and ultimately become an elegy for a generation of lost youth. Here the Pictish “barbarians” have lost the elders of their tribe. They’re courageous, fierce, struggling to be mature but are unfocussed until the considerably senior Doctor and Nardole intervene. The Roman soldiers are all teenagers. The eldest nicknamed Grandad is only 18. (I have to say they all look older.) They’re cowed, on the run, rudderless, until Bill comes among them and inspires them.

Bill is the trigger to the tale. The Munro avatar. She wants to investigate a mystery that has puzzled her since school: the disappearance of the Ninth Legion in ancient Caledonia. She achieved an A* in the subject of Roman Britain. She blunders into danger, finds the last vestiges of the legion and solves the puzzle. Along the way, she has fun acknowledging the Time Lords’ almost universal gift of translation (“Oh my God, it even does lip-synch”) and discusses sexuality with the Romans, realising that of course they’re refreshingly broadminded.

Peter Capaldi’s Doctor (“I’ve lived in Roman Britain, governed, farmed, juggled”) does what he does best, taking authority, throwing himself into danger none too seriously but willing to sacrifice himself or a chunk of his being. Here the Time Lord is determined, eager to be the gatekeeper against the baddies. “It’s who I am. I’ve been standing by the gates of your world, keeping you all safe since you crawled out of the slime. I’m not stopping now.” It’s almost poetic justice that the Old Man of Time is stood down, knocked down even, by the youth of the day, Roman and Caledonian, who unite to stave off the enemy. Theirs is a sacrifice that will echo down the centuries.

Nardole realises that “the crows aren’t sulking. The crows are remembering.” Their mournful call isn’t caw-caw but “Kar, Kar”, recalling their Pictish saviour. I’m a sucker for anything that elevates or mythologises birds. Face the Raven in 2015 may not have quite hit the spot, but I’m instantly reminded of the watchful crows (“eyes and ears” of the Cailleach and possibly the Black Guardian) in my favourite Tom Baker story, The Stones of Blood (1978). Stone the crows! That’s two great scripts with crows and standing stones.

Nardole is fully integrated into the narrative for once. “I’m blending in,” he claims, despite his garish bobble hat and dressing gown. He’s such an engaging oddball, and Matt Lucas plays the lighter, comedic notes so well. (“This is worse than jazz.”) Nardole spins yarns for the locals and befriends them sooner than the Time Lord. “It’s called charm,” he says. “I’m against it,” says the Doctor.

The monster of the week almost doesn’t matter. Skulking, then rearing up like some fiend from Celtic or Norse mythology, it’s reasonably alarming and fairly well realised in CGI. “Every hour of sunlight that feeds it makes the world darker and the beast stronger,” explains the Doctor. It has beset the locals for generations and laid waste the marauding Romans. Its victims die by a “complete and total absence of any kind of sunlight”… “Death by Scotland,” Nardole drily notes. I have relatives who’ll empathise.

With a youthful, largely unknown but skilled guest cast, slick direction from Charles Palmer and evocative soundtrack from Murray Gold, Rona Munro completely draws me into her world for 45 minutes or so. As the crows fly, Bill, Nardole and the 12th join the Picts and the Ninth against the hounds of hell. That’s the stuff of legend.