In the first sentence of the first chapter of this book, Henry Marsh, a consultant brain surgeon, says, ‘I often cut into the brain and it is something I hate doing.’

What a compelling start! Marsh takes us through an operation. First, he looks at a scan. A middle-aged man has a tumour of the pineal gland, which means a very tricky operation. Brain surgeons, says Marsh, look at pineal tumours ‘with both fear and excitement, like mountaineers looking up at a great peak they hope to climb’. And with this, he sets the tone for the book, which is excellent. For Marsh, brain surgery is terrifying, but also addictive. You become obsessed. Cutting people’s heads open, it turns out, really does your head in. On the positive side, the fear and the self-doubt usually fall away once the operation has begun. But sometimes, horribly, they don’t.

Now for the pineal tumour. Marsh talks to the patient the night before. It’s easier for the patient if you’re positive and reassuring, but stressing the downside takes the pressure off the surgeon. A difficult balance. In this case, he’s still tormenting himself about the last operation that went wrong; he accidentally paralysed a woman down one side. And he can’t work out how it happened: ‘The operation had seemed to proceed uneventfully.’ Now he’s really desperate for the next one to go well.

So desperate, in fact, that he gets ‘severe stage fright’. He needs to cut blood vessels inside the man’s brain — but which ones? It’s not unlike bomb disposal. One false move and he’ll be lost in a torrent of blood. He snips away, pushing down his anxiety. Afterwards, he needs to have an agonisingly awkward bedside conversation with the woman he has accidentally paralysed. ‘I trusted you,’ says the woman. ‘I had no immediate reply to this,’ Marsh tells us.

As a young doctor, he had felt ‘disappointed and disillusioned’. But one day he was asked to help with a brain operation — the ‘clipping’ of an aneurism. He says: ‘It was more like a blood sport than a calm and dispassionate technical exercise’, adding, ‘It was love at first sight.’ That night he went home to tell his wife he’d decided on a medical career. What he writes next is perhaps the key to the whole book, and indeed to the whole of Marsh’s life:

Neither of us could have known then that my obsession with neurosurgery and the long working hours and the self-importance it produced in me would lead to the end of our marriage 25 years later.

So this, in a way, is about a man having an affair; the thing is, he’s having an affair with brain surgery. Like an amour fou, it always contains danger and mystery — things go wrong suddenly, and he doesn’t know why, and the only cure is to do more brain surgery. He sits there, expertly poking instruments into something he can never quite understand, never quite master. Sometimes he’s filled with joy; sometimes he’s overpowered with morbid gloom. He can’t relax. He must get everything right, ‘the surgical drapes placed in exactly the right way, the instruments tidily laid out’. And then he must probe his instrument ‘through thought itself, through emotion and reason’. After this, he cycles home to his wife.

He takes us through the gamut of brain operations. It’s hugely compelling. There’s a haemangioblastoma — a tumour of the blood vessels. There’s a mengioma, in which the brain’s outer sheath becomes cancerous. Horribly, there’s a neurotmesis — the severing of a nerve. In this last case, he delegates a simple spine operation to a junior, who screws it up. ‘Oh Jesus fucking Christ! You’ve severed the nerve root!’ says Marsh. Desperate, he wonders if he should have done the operation himself. But then, if surgeons never delegated to juniors, there would be no experienced surgeons in the future. In this moment, he thinks: ‘Fuck the future.’ And then: ‘Fuck everybody.’ You can see how it does your head in.

Luckily, the pineal operation goes well. ‘It went as well as we could hope,’ says Marsh to the patient’s wife,  ‘playing the part of the detached and brilliant brain surgeon’. Having said this, he finds he needs to control his tears. And then go home — to his second wife.