Smiley hesitated. "Come and dine with me that evening . . . Quarter to eight. Bywater Street, Chelsea, number 9A." Fielding wrote it down in his diary. His hand was quite steady. -- from "A Murder of Quality," by John le Carre'

Some real-life places belong to fictional characters. No. 7 Eccles St., Dublin, is physically no more than a door in a wall, but generations of English majors remember it as the home of James Joyce's Leopold Bloom. A hundred years later, London postmen still deliver mail addressed to Sherlock Holmes, 221B Baker St.

Perhaps the relative changelessness of the British Isles leads their authors to choose real settings for their stories. Certainly the tradition continues; and one of the latest writers to follow it is John le Carre', author of such espionage novels as "Tinker, Tailor, Soldier, Spy," "Smiley's People" and "A Perfect Spy," published just this month.

Le Carre' has set his novels in places as diverse as Indochina and Beirut. "A Perfect Spy" even takes in Washington, with a clandestine rendezvous among July Fourth crowds on the Mall and dead-drop message exchanges in Georgetown's Oak Hill Cemetery. Three cities, however, figure most often in his books. One is Berlin. Le Carre''s first spy novel, "The Spy Who Came In From the Cold," began there, exactly where "Smiley's People" ends: with tired agents standing on the western side of the border, hoping the man they wait for will make it through the searchlights and barbed wire. Another is Oxford, where a dying woman delivers -- in "Smiley's People" -- le Carre''s verdict on espionage: "Half-angels fighting half-devils. No one knows where the lines are." But the city that appears most often is London: capital of post-Empire Britain, center of British Intelligence, home of master spy George Smiley.

Le Carre''s novels share a solid factual grounding. (As they should: Their author, before the success of "The Spy Who Came In From the Cold" freed him to write full time, worked within the British intelligence community.) His characters are fictional, but they live and work at real locations. Walking from one spot to another, as Smiley prefers to do, provides a morning's tour of London. Officially, Britain's secret services are known as Military Intelligence. Smiley's branch, the Secret Intelligence Service, is known as M.I.6. Throughout the Smiley novels, le Carre''s name for the Secret Intelligence Service is "the Circus." A circus, in terms of British urban geography, is a major intersection. The Circus is named for Cambridge Circus, a traffic circle in central London. In the world of le Carre''s fiction, that is where its headquarters are.

Cambridge Circus marks the northern end of London's theater district. Streets fan out spokelike from it toward the British Museum, Piccadilly Circus and Trafalgar Square. At its center is a concrete circle that contains an unlighted lamppost and a locked-up public restroom. Heavy Victorian buildings line the intersection, their ponderous turrets and bay windows staring down on a steady stream of traffic.

Le Carre' has never revealed directly the location of Circus HQ. Hints scattered throughout the novels, however, suggest that it occupies a triangular building -- half hidden by hardy city trees and labeled "Bill Lewington -- Musical Instruments" -- whose narrow end barely touches the traffic circle's northeast corner. Smiley's office would overlook the Cambridge Theatre, where "Jesus Christ Superstar" first opened. One can imagine him at work: a short, heavy, aging, unglamorous man, cleaning his glasses on the end of his tie. His desk would be littered with folders and paper; he would write, slowly and deliberately, on single sheets of paper laid on a glass plate -- leaving no trace of his work. On the wall behind him would be his photo of Karla, his Russian counterpart and antagonist.

Directly across Cambridge Circus, another theater advertises itself with crimson stars and draperies. The effect is to give Cambridge Circus a touch of Red Square. Doubtless Smiley would have noticed it; doubtless it would have appealed to his sense of irony.

On a quiet stretch of Curzon Street, as far southwest of Piccadilly Circus as Cambridge Circus is north of it, stands a very large, unlabeled building -- with no windows on the ground floor, one very solid-looking door and antennas scattered along the roof. This may be the actual headquarters of Britain's M.I.5 -- the Security Service, which is responsible for internal counterintelligence. This is name may be Leconfield House. (The location of important defense sites is closely guarded under Britain's system of D-notices, which forbids the publication of such information.) But whether it is based in Leconfield House or elsewhere, M.I.5 headquarters does figure in le Carre''s fiction. When Smiley first appeared -- in le Carre''s first novel, "Call for the Dead" -- he was with M.I.5, serving a Cold War stint in counterintelligence.

Walking home from the Circus -- slowly, but with method -- Smiley might pass through Curzon Street. He would keep bearing southwest, cross Piccadilly and walk down Knightsbridge. Hyde Park Corner, where England's soap-box orators vent opinions outrageous and eccentric, would be on his right. Then Smiley would turn left along Sloane Street, turn right at Sloane Square. Then he would be on King's Road, almost home, on the fringe of Chelsea.

Chelsea has a well-earned reputation as the trendiest part of London. Posh and punk mingle here. Harrods is a five-minute jaunt away. Diana, princess of Wales, in the days when she was a mere kindergarten teacher and earl's daughter, shared a flat in the neighborhood. So many brisk, upper-crust, tennis- and riding-minded young Englishwomen frequent Sloane Square that the type has become known as the Sloane Ranger. They share the streets with the other half of British youth: teen-agers with spiky neon-colored hair, boys and girls dressed alike in button-studded leather and denim. And strolling past the rows of boutiques are members of a third British subculture. These are old men in dark blue uniforms and caps, their tunics brightened with campaign ribbons: pensioned veterans from the Chelsea Royal Hospital.

Bywater Street is a block-long, dead-end lane leading off King's Road. It looks ordinary. Two rows of town houses, their porches ornamented with iron railings, face each other across a quiet, car-lined street. Perhaps le Carre' chose it because of its name. For Smiley, forever being called from retirement to run espionage missions or hunt double agents, "Bywater" is a fitting address.

In another sense it is a bywater: As one rounds the corner, the bustle of King's Road ceases. Both sidewalks are empty. The only persistent sound is the pecking of an unseen typewriter. A young woman and a little boy, both in jogging suits, appear at one porch. "Number 9A?" she says. "That'd be across, over there. The gray house. They're always filming there."

No. 9 is a house like all the other town houses. All that distinguishes it from its neighbors is a coat of gray paint -- medium gray, the shade the English call "dove grey." At an upstairs window there is a hint of curtains: white with a brown, ornate pattern. The pattern would be too rich for Smiley. It would fit his faithless wife Ann, who carries on her adulteries with aristocratic coolness.

The doorbell buzzes, but no one answers. No one answers the knock. Nothing moves at the window. The panels of the door are lined with dust. Whoever lives now at 9A Bywater St., he (or she) has been away some time.

Around Bywater Street, surrounding Curzon Street and Cambridge Circus, lies a living city. London's roof lines have not changed in a century; Dickens would recognize their turrets and quaint Victorian gables. He would not recognize the storefronts, where restraint and dignified austerity have been replaced with plastic signs and orange stickers proclaiming the latest bargains.

London no longer seems a purely British city. Chinese and Indian restaurants, dishing out fried rice and curries to go, encroach on the tradition of fish and chips. Black-veiled women huddle outside phone boxes. The Empire has come home to London: Indians, Chinese, Jamaicans, Arabs. And Czechoslovaks, and Estonian defectors, and Danzig Poles who speak German -- the people with whom Smiley works.

A century ago, the British Empire was defended by intelligence work -- the mix of intrigue, reconnaissance, analysis and action that Kipling knew as the Great Game. On India's borders, British agents sent out spies and pondered the loyalties and information they dealt with -- trying to second-guess the intentions of Russian agents. The Empire's homecoming has not changed the pattern. The Great Game continues, with fresh players -- on home terrain, in le Carre's London.

But time and reality do have a way of changing things: 221B Baker St. is now a bank. "A Perfect Spy" shares no characters with le Carre''s earlier fiction; "the Circus" has even been renamed "the Firm." The chill world of espionage seems a little more bleak with Smiley not present to sort things out. But as Holmes survives in the imagination, Smiley is still there to be recalled, from the room beyond the locked door, where he sits completing his monograph on 17th-century German literature.