Heaven Sent Doctor Who episode satisfies this SF critic at last: five-star review

WARNING: SPOILERS AHEAD

It is always good to be brought into the light, even though it may take an age: better late than never. And so the theme of Heaven Sent, the penultimate episode in the latest series of Doctor Who, brings me to my knees in grateful awe. Heaven Sent knocked me off my cynical perch where I've been nailed ever since Russell T Davies rebooted the Time Lord franchise to dreary derivative effect. And may I say I have never been happier to have had my opinions reversed so totally even if this turns out to be one glorious, single, solitary, diamond-perfect episode before it all goes back to normal.

Quite staggering in its concept and clever in its execution, this episode never makes a wrong turn. Blessed relief to find histrionics kept to a minimum, only reflecting the Doctor's impossible predicament and thus earning him the right to emote furiously, epically and truthfully. At last, Steven Moffat gives Peter Capaldi a script worthy of his talent and our expectations.

A grieving Doctor is deposited in a vast unrealisable castle, who-knows-where, which turns out to be a gigantic puzzle reminiscent of the classic The House That Jack Built episode (1966) that had Diana Rigg's Emma Peel so thoroughly trapped and beaten in The Avengers series.

Pursued relentlessly by The Veil, a monstrous shade of Death, it is only when the Doctor solves the first bit of the puzzle that we realise how difficult this challenge is. He is forced to struggle across all his levels of existence, from his deepest inner nightmares, to engagement with the ghost of Clara, to his survival on a colossal cosmic scale, and is tried to his utmost ingenuity and courage.

"How many seconds in eternity?", he asks. Not such an empty question as you might think.

A truthful confession dredged out of his deepest recesses stops The Veil in its tracks and resets the castle, whose floors and rooms rotate and move, for the next stage of the chase. Here, even the constellations are all wrong: the stars tell him that his very own torture chamber is 7,000 years in the future. How long has he been playing this game? Will he work out his ultimate escape? Will he run out of confessions?

Piece by piece he solves the puzzle and escapes death time and time again. Who was the owner of the skull he finds at the top of the tower still attached by electrodes? What is the meaning of the word written in the dust? He discovers that the castle is an island surrounded by a sea of human skulls. What cruelty has the castle's creator unleashed on these poor souls?

And yet, as in all the best stories, beneath the byzantine puzzle, there is a simple explanation whose driving force shoots this episode to the front of the SF screen rankings.

Our Doctor finally reaches the Home room, the square at the end of the game, where the Tardis and escape is set tantalisingly the other side of a crystal wall. At 400 times the hardness of diamond and twenty feet thick, it is impossible to penetrate. And yet, this surely has to be the moment he vanquishes? Well, yes, but not in the way you might expect.

What follows is a tour de force sequence that satisfies every demand for great story-telling. The moment you realise how the Doctor is going to win takes your breath away and is beautiful in its simple, profound truth. Here is eternity in a grain of sand, heaven in a flower. You could almost call this Triumph of the Will, but let's not go there.

So, bravo, Steven Moffat. It didn't take you a billion years to get here after all. It only felt like it. Salud.

Where to watch Heaven Sent.
On iPlayer.