Memento

Leonard Shelby used to investigate claims for an insurance company. Then, one night, intruders broke into his home. His wife was attacked and killed; Leonard was struck hard on the head. He survived — but since that night he has been unable to keep anything new. His old life is perfectly intact: his childhood, his job, his wife's face, the sound of her voice. Everything after the attack lasts a few minutes and then simply dissolves. He walks into a room and cannot say why he came, or how long he has been standing there.

So he builds a system, and he is meticulous about it. Polaroid photographs of every person, car and building that matters, each one annotated in his own handwriting. Notes folded into his pockets. And, for the facts he refuses to risk losing, tattoos across his chest, his arms and his legs — a numbered list of what he believes he knows about the man who killed his wife. His body has become his filing cabinet.

Nolan tells the story in a way that puts you inside the condition rather than watching it. The colour scenes run in reverse order: each one begins where the previous one ended, so you arrive, exactly as Leonard does, in the middle of a situation with no idea how you got there or who anybody is. Running between them, in black and white, is a second strand that moves forwards — Leonard alone in a motel room, talking on the telephone about a former insurance client called Sammy Jankis, a man he says had the same condition. The two strands meet at the end of the film, which is the beginning of the story.

Leonard is hunting a man he knows only as John G. Two people attach themselves to him. Teddy is friendly, talkative, endlessly helpful, and always seems to know more than he is saying. Natalie works in a bar; her partner has disappeared, and she has problems of her own. Both of them are warm to Leonard. Both of them are steering him.

In the film's most uncomfortable sequence, Natalie works out precisely how weak he is. She insults his dead wife until he loses control and strikes her; then she walks calmly outside, waits for the memory to disappear, and comes back in as his friend and ally — bruised, and blaming somebody else. He believes her instantly, because he has nothing left to check her against. It is not a violent scene so much as a chilling one: we watch a person being edited.

Guided by information he has been fed, Leonard tracks down and kills a man called Jimmy Grantz in the basement of an abandoned building. Almost immediately, small details — the dead man's clothes, his car, a name he said before he died — stop fitting the story Leonard has been told. Something is wrong, and Leonard has no way of reconstructing what. Because the film is running backwards, we have already seen the consequences of this killing before we understand its cause.

Cornered, Teddy finally tells Leonard what he says is the truth. He is a police officer. Leonard found and killed his wife's real attacker more than a year ago, with Teddy's help — Teddy even took the photograph. Since then, Teddy has simply been recycling him, pointing him at a new "John G." whenever it suits, because Leonard is a weapon that forgets it has been fired.

And Sammy Jankis, Teddy says, is a story Leonard has partly rebuilt. In Teddy's account, Leonard investigated Sammy as an insurance claim, decided his memory loss was psychological rather than physical, and refused to pay. Sammy had no wife who died of an overdose. It was Leonard's own wife who survived the break-in — and who later died after asking, again and again, for the insulin injections that Leonard, unable to remember giving the last one, kept giving her. The grief in the story is real; the name on it may not be.

Whether Teddy is telling the truth is left deliberately open. What is not open is what Leonard does next. He hears the facts, decides he cannot live inside them, and destroys them: he burns the photograph, writes himself a false note, and deliberately adds Teddy's number plate to the list of "clues" tattooed on his body — knowing that a future version of himself will read it as proof and go hunting again. For about ninety seconds he understands everything. Then he chooses, consciously, the world he would rather wake up in.

Henry Gustav Molaison was born in Connecticut on 26 February 1926. He developed epilepsy in childhood, possibly after a bicycle accident at seven; by his late twenties the seizures were disabling despite very high doses of medication. On 1 September 1953 the neurosurgeon William Beecher Scoville performed a bilateral medial temporal lobectomy, removing structures on both sides of his brain including most of the hippocampus, the entorhinal and parahippocampal cortex, and the amygdalae. It was experimental. It was also, in a way nobody foresaw, catastrophic.

The seizures improved. But Molaison could no longer lay down new memories at all — his span was around thirty seconds. He kept his intelligence, his manners, his childhood and his sense of humour, and he lost every day as it happened. He was known to the world only as "H.M." until his death on 2 December 2008; his full name was published only afterwards. He lived for many years in a care home in Windsor Locks, Connecticut, and took part in research for over half a century — never once able to remember having done it before.

From 1957 the neuropsychologist Brenda Milner studied him, and made the discovery that split memory in two. She gave him a mirror-drawing task: trace between the lines of a star while seeing only its reflection, which reverses every movement your hand makes. Day after day his accuracy improved, exactly like any healthy learner — and every single time, he insisted he had never attempted it before. Something in him was learning. Nothing in him was remembering. After his death his brain was preserved, cut into 2,401 thin slices and reconstructed as a digital atlas that was made freely available to researchers.

Clive Wearing, born on 11 May 1938, was a distinguished English musicologist, conductor, tenor and keyboard player. He had been chorus master of the London Sinfonietta, worked in music broadcasting, and specialised in Renaissance music, editing the works of the sixteenth-century composer Orlande de Lassus. On 27 March 1985, in his mid-forties, he contracted herpes simplex encephalitis. The infection destroyed his hippocampi and much of his medial temporal lobes.

What was left was a memory measured in seconds — somewhere between about seven and thirty. His carers gave him a diary. He filled page after page with the same discovery: that he had just this moment become conscious for the first time, each earlier entry crossed out as a mistake, hundreds of near-identical pages, still being written more than twenty years later. When his wife Deborah comes into the room he greets her with overwhelming joy, as though after years apart — and does so again if she steps out for a minute. She wrote about their life in her memoir Forever Today (2006), and the neurologist Oliver Sacks described him in a 2007 essay, "The Abyss".

And yet: sit him at a keyboard and he plays. He sight-reads music he cannot remember seeing, performs complex piano and organ pieces, and conducts a choir with the authority he had before the illness — Sacks described his musical powers as essentially intact, and his playing as still full of feeling and structure. For the length of a piece of music he is not a patient. When the last chord dies, so does the memory of having played it.

Molaison and Wearing demonstrate the same split. Declarative memory (also called explicit) is the memory you can put into words: facts, and events that happened to you. It depends on the hippocampus and the surrounding medial temporal lobe, which is why damage there destroys it. Procedural memory (also called implicit) is the memory in your hands: skills, habits and conditioned responses, learned by doing and repeating. It runs on different machinery, and it survives. H.M.'s mirror drawing and Wearing's Lassus are the same phenomenon in two different lives.

Memento is unusually well built on this. Leonard keeps his identity and his whole pre-injury life, as real patients do. His everyday failures are the real ones: losing the thread mid-sentence, finding himself running and not knowing whether he is chasing or being chased. And the film's central plot device is scientific rather than invented — Leonard's insurance test of Sammy Jankis, picking up objects of which some are electrified, is a version of a genuine clinical logic. A person with organic amnesia would learn, without any conscious recollection, to avoid the ones that hurt. That is procedural learning, and it is why Leonard himself can be conditioned, shaped and steered by people all through the film without ever knowing it. The neuroscientist Christof Koch has called the film the most accurate portrayal of the different memory systems in popular media.

Where it dramatises. First, the cause: a single blow to the head producing pure, permanent, isolated anterograde amnesia is very rare — the real culprits are infection, stroke, oxygen loss and surgery. Second, and much bigger, the independence. Leonard drives, rents rooms, dresses well, holds conversations and runs a multi-day investigation alone. People with damage that dense need constant care; Molaison lived in an institution, and Wearing has been looked after for four decades. Third, the note system. Wearing's diary is the real-world version of Leonard's Polaroids and tattoos, and it does not work as a substitute memory — it records the same "first" awakening over and over. To use a reminder you have to remember that you have reminders, and that is precisely the part that is broken. Leonard's elegant, functioning archive is the film's most beautiful lie.

One last real detail. The film came from a short story, "Memento Mori", written by Christopher Nolan's brother Jonathan Nolan and published in Esquire in March 2001 — after the film had already premiered at Venice in September 2000. In the story the character is called Earl, and he lives in an institution, which is rather closer to how this actually goes.