Ian McEwan may now be the best novelist in Britain — and is certainly operating at the height of his formidable powers. Famously, McEwan made his name meditating coolly on the macabre. His precise, taut prose cuts clean as a scalpel, and his forensic intelligence addresses steadily the deepest of human horrors: incest, murder, psychosis, and so on. Yet in the wake of the 9/11 terrorist attacks, he has decided to do something very different: to write about happiness.

The contingent, delicate, transient nature of happiness is best appreciated not by novelists but by members of the medical profession. Novelists might sit about thinking, obsessing and making things up — but doctors see real lives fall to pieces in their consulting rooms or on their operating tables, day in, day out. Often they mend what is broken, and open the door to happiness again. Many doctors have neither the time nor the inclination to read fiction — lots of them say they don’t need it.

McEwan dared to cross this divide. First he spent two years job-shadowing a neurosurgeon in a London hospital; then he made a neurosurgeon the central character in his new novel. The result is a book of great moral maturity, beautifully alive to the fragility of happiness and all forms of violence — chemical, biological, social and political — threatening it. Everyone should read Saturday, doctors definitely included.

The novel is set on Saturday February 15, 2003, the day of the immense anti-war demonstration, which failed to change the Government’s policy on Iraq. On this day, Henry Perowne wakes in the early hours, looks out of his expensive window, and sees a plane on fire streaking towards London. Horrified but riveted, he reflects: “Everyone agrees, airliners look different in the sky these days, predatory or doomed.”

A few hours later, after the plane has landed safely at Heathrow, the flames extinguished and no one hurt, Perowne registers the details that he ignored to nourish his fears — it was almost as though he wanted to believe the worst was about to happen. A little confused, but no longer really disturbed, he returns to bed and makes love with his beautiful, clever wife, Rosalind, to whom he has always been effortlessly faithful throughout their stable marriage. “This fidelity might look like virtue or doggedness, but it’s neither of these because he exercises no real choice. This is what he has to have: possession, belonging, repetition.”

Choice, inevitably, introduces the possibility of human error. So in order to guarantee the happiness of Perowne’s marriage, McEwan removes some of his freedom. The underlying picture of humanity is bleak: free people with choices are sure to mess up. This is a comment on personal relationships, but equally a comment on democratic politics.

After daybreak, the assembling protesters fill the streets, and the police begin to play their part in the stylised drama — mostly diverting traffic with wry expressions on their faces. Perowne reads the banners as they go by and notes the “cloying self-importance” of: Not in My Name. Amusingly, but also very intelligently, he prefers the more languid: Down With this Sort of Thing. Even though Perowne is at the top of his own profession, he is modest in the face of political complexity. “In neurosurgery he chose a safe and simple profession.” Politics, and the study of politics, is more difficult.

For these reasons, Perowne, and behind him McEwan, is reluctant to settle on a definitive opinion on the moral rights and wrongs of the war. Perowne once treated an Iraqi academic who had been tortured under Saddam’s regime. Chillingly, he then interested himself in what happened to the Iraqi surgeons who refused to carry out the bizarre punishments and amputations passed into law by Saddam. Wisely, he reflects that if the protesters really think a regime of terror preferable to invasion, they should be marching more sombrely in London streets.

Perowne does not join the march. Instead, he tries to get to his regular weekend squash game on time. In the process, he has a minor car accident and becomes embroiled in a violent dispute with the less genetically and socially privileged driver of the inferior vehicle with which his own Mercedes has collided. Like the background march, this encounter is heavily stylised: “The unrelenting throb of drums is not helpful to the situation, and the fact that so many people are close by, unaware of him, makes Henry feel all the more isolated.”

Saturday develops into a sinuously plotted drama conforming to the classical unities of time, place and action. It also includes a series of brilliantly vivid tableaux, in which McEwan slows or freezes the dramatic action to better emphasise the rituals of daily violence that surround, and sometimes even protect, our elusive experience of happiness. Artistically, morally and politically, he excels.

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