ENDURING LOVE.<i> By Ian McEwan</i> .<i> Nan A. Talese/Doubleday: 245 pp., $23.95</i>

It begins with dazzling cinematic bravura. Joe and Clarissa, his live-in lover, are having a spring picnic in Britain’s Chiltern Hills. About to uncork the wine, they hear a man shout.

“We turned to look across the field and saw the danger. Next thing I was running towards it,” Joe relates. Then for an instant, like a bright lure seized by a bottom fish and dragged under, the dazzle yields to leaden hindsight. “What idiocy, to be racing into this story and its labyrinths, sprinting away from our happiness among the fresh spring grasses by the oak.”

Narrator’s hindsight and reader’s foreboding: Ian McEwan weights them on before surfacing back into the immediacy of what is taking place. “There was the shout again, and a child’s cry, enfeebled by the wind that roared in the tall trees along the hedgerows. I ran faster. And there, suddenly, from different points around the field, four other men were converging on the scene, running like me.”

McEwan, author of “The Cement Garden” and “The Child in Time,” shapes this dramatic opening with breathtaking dexterity. Alternating between Joe’s dazed, camera-like perceptions and his subsequent settled narration, we get the transformation of picnic idyll into disaster.

A balloon carrying a child and piloted by his uncle had landed in the field without incident. Getting out of the basket, the uncle tangled his legs in the lines, and a sudden gust began dragging the balloon, the child and the uncle to the edge of a promontory falling steeply to the Oxfordshire plain hundreds of feet below.

Joe and the four other men who spot the danger manage to grab the lines. They work at confused cross-purposes, though, and a violent updraft begins lifting them into the air. Had they all held on they might have maneuvered the balloon down, but one panics and lets go. Feeling themselves jolted upward and afraid of being carried off, Joe and two others release their hold, falling a dozen feet.

Only one, a doctor named Logan, holds on, counting on the others until it is too late. At 300 feet, his grip loosens, and he plummets to his death. (The balloon eventually drifts down, and the child in the basket is unharmed.)

McEwan, one of Britain’s finest writers, renders it all with a characteristic skill: peacefulness changing in a flash to panicked horror and then to shock’s slow, disabling tide. It is bound together by a grim moral reflection on how panicked animal reflexes instantly obliterate social cooperation. “Someone said ‘me’ and then there was nothing to be gained by saying ‘us,’ ” Joe says.

It is a wild racehorse of a start. Immediately, though, McEwan hitches it to a dray cart. Joe’s initial words about “sprinting away from our happiness” do not refer to the tragedy and its consequences for him and Clarissa--consequences on whose exploration we seemed to be launched in such fire and style. It refers to an entirely tangential development.

One of Joe’s fellow would-be rescuers is a weird young man, Jed Parry. A solitary, possessed of a certain amount of money, he is convinced that he has a vaguely Christian religious mission. His deep, wasting mental disorder, though, is the conviction that Joe has fallen in love with him and that it is their destiny to be together.

Most of the rest of “Enduring Love” tells of Parry’s obsessive pursuit and--as if the sick mind spreads contagion--of Joe’s near undoing, both in himself and in his relationship with Clarissa. It is a creepy story and told with skill, but its mismatch with the beginning is disconcerting. This is true not just because it wanders narratively but because of its decidedly inferior level of fictional energy and expansiveness.

Parry phones repeatedly, stations himself in front of Joe’s front door, pursues him down the street weeping. If he were simply proclaiming his love, it would be bad enough; the true horror lies in his insistence that Joe loves him, has incited him and simply feigns indifference in order to exercise power. Parry leaves as many as 33 messages in the course of a single day. He writes long letters to Joe, picturing their life together and urging that they sit down with Clarissa to work out a fair arrangement.

Nothing could be worse, of course, than to be conscripted into the mind of a solipsist, particularly one with a deranged obsession. It is like being pursued by an enormous molasses jar, lid flapping hungrily. For Joe, who has his own insecurities--a sense of failure at throwing away a career in science to become a best-selling popularizer--the siege is disabling. Particularly it disables his relations with Clarissa, and McEwan writes a masterly scene in which their two sets of jitters go into a fibrilation that comes close to being lethal.

Parry’s pursuit goes from entreaty to threat, and there is a series of melodramatic scenes of near-lethal violence before the police are persuaded to take action. At one point, Joe visits a household of former hippie drug dealers to buy a pistol; the scene has its piquancy, but it seems quite detached in style or effect from everything else.

McEwan is unable to be dull, and his detail is invariably sharp and well-managed. But “Enduring Love” is seriously flawed. The author has researched a psychological disorder known as De Clerambault’s Syndrome, named after the author of a study of a Frenchwoman who haunted the sidewalk in front of Buckingham Palace, convinced that King George V was sending her love signals. One signal, she claimed, was a periodic twitching of the upstairs curtains. Twitching curtains, in fact, are part of Parry’s delusion.

The author, in effect, has written a fictionalized version of a case history, which he appends at the end in the form of a study printed in a psychiatric journal. The study is itself made up, although the reader may not realize it, as I did not until a perceptive editor checked up.

This is confusing, to say the least; furthermore, why invent a case history to bolster a story of manic obsession unless the author is unsure of making it convincing? Until the melodramatic ending, in fact, Parry’s obsessive pursuit is credible, however grisly.

Confusion apart, to turn “Enduring Love” into a tale of a sick mind and the contagion it spreads is to narrow sharply the beginning’s expansive promise of a fictional and moral exploration with an open road in front of it.

Instead, the reader is shunted indoors to something much less: a storified lecture, graphic and well-illustrated.