When I'm reviewing a programme I'll normally watch it twice: once just for interest and then a second time with my pen, paying attention and picking up quotes. So I have to admit I only watched Inside No 9 (BBC2) once, and that's because I couldn't bear to watch it a second time.

Each episode of Inside No 9 has a different flavour: one week it might be a viciously black comedy, and the next week they'll maybe offer a sinister story, so it's easy to slot the series in with Tales of The Unexpected or The Twilight Zone, but did either of those series ever make you cry and then curl tight around your mind so you woke up twice during the night to cry afresh? Probably not, and so with tonight's brilliant episode, The 12 Days Of Christine, Shearsmith and Pemberton show us that they can't be neatly placed alongside other anthology series of weird tales; they stand above all others, and The 12 Days of Christine must be their finest.

Sheridan Smith plays Christine. She stumbles home from a New Year party, drunk and dressed as a kinky nun. She's brought Adam back with her, and they have a passionate night beside the flashing fairy lights. A relationship blossoms. Soon they're moving in together and Christine is introducing Adam to her mum. The story speeds up, and each scene jumps us forward in Christine's life: she's pregnant. Her baby son comes home. Her dad dies. Her son gets older. Christine and Adam argue. Christine and Adam break up. Christine cries: life is not what she hoped it would be. Christine panics: her son has burnt his hand on a firework.

But has he? Because Christine starts to get confused. She runs cold water over his burned hand, but her mother appears and tells her she's wrong. Look, she says, his hand is fine. Poor Christine is mixed up, she says. She must be thinking of a time, years ago, when she burned her own hand on Bonfire Night.

Her confusion deepens when she starts to have strange visions of a man in her flat, a man wearing a wet raincoat, who looks panicked and afraid. There's one terrifying scene where eggs are being smashed on her kitchen floor and hurled against the walls. The man appears, soaked and breathless, and he keeps insisting he's sorry, he's sorry!

Who is he? Christine doesn't know, and neither do we. She's confused. 'I'm getting everything jumbled up,' she says. Her family arrive for her birthday, but then it's suddenly Christmas dinner and her dad is with them, but didn't he die? I was confused, but so was Christine. Suddenly Adam is at the dinner table. We look at him, squinting at him, frowning, as does Christine. She asks him why he's here. 'Adam, are we back together?' She can't understand what's happening, and we're forced to share her confusion.

Round the table, everyone is leaning forward, looking at her. Why are they staring? Then her mum says it's time for her present. 'I don't want the present!' says Christine.

Her mum gently hands her a photo album and she flicks through it, laughing softly at the old pictures of her as a baby, as a toddler, as a teenager. She jokes that it's like having your life flash….She stops and looks up. Her quiet confusion vanishes and a terrible, wordless fear takes its place.

'Oh,' says Christine. 'I think I know what this is now.'

Blue lights flash and whirl and Christine is slumped onto the steering wheel. Her eyes flicker open and she sees someone try to cut her free from the car. Her shopping on the seat next to her has been thrown forward and the broken eggs drip watery yolk onto the floor. Outside a man in a raincoat keeps telling the police he's sorry! He's sorry! He just walked out without looking. He's sorry!

There was no horror. We never saw the crash which killed Christine. We just saw her drifting, flickering, fading on the steering wheel. Her eyes flutter open and closed as her brain shuts down and her life reels out before her in a succession of jumbled scenes and memories. And there was no thunderclap moment when the story's twist is spectacularly revealed. There was just the slow and terrible realisation which we shared with Christine. We were with her, thinking 'Oh god no, not that. Don't let it be that!' That's the wonderful thing about a one-off, 30 minute story: you can be totally immersed in one character's confused and flawed point of view. We were standing at Christine's side throughout and I'm sure I felt my hand tighten on her shoulder when we both realised what was happening: oh no, not that!

The writers so cleverly threw us off the scent, making the eventual realisation so agonising. The first odd thing Christine experienced was a Valentine's Card from an old boyfriend, and she wonders how he got her address. That suggested the story might be about an obsessive ex, a stalker, perhaps? Then, with Christine's tears and confusion and her degenerating physical appearance, the idea is planted that she may be suffering some kind of mental collapse. At no point did we imagine Christine was dying in a wrecked car, her brain slowly fading, her memories blinking out, light by light, into darkness.

Bravo to every single person involved in this incredible show.