KEN FOLLETT is a master of appropriate settings. He seems to have sensed that the Cold War has about run its course as a fictional genre. The climax came when John le Carre, in Smiley's People, brought Karla, the Soviet master spy, across the nocturnal landscape to Berlin to surrender. The Cold War setting for spy-and-terrorist fiction seemed used up thereafter.

Follett worked that territory--and its Middle Eastern suburbs--in his second and third novels. Now, in his fourth, he has returned to the genius of his first, Eye of the Needle. If the present palls, half-remembered periods of history do splendidly.

The Man from St. Petersburg takes place in London on the eve of World War I. Thrillers should shatter comfortable assumptions with alien presences. The assumptions of the belle ,epoque cried out to be shattered; history is a pendulum and if the Great War hadn't happened somebody would have had to invent it.

The focal family of Follett's new book is made up of a titled trio--Stephen, Earl of Walden, his Russian-born wife, Lydia, and their 18-year-old daughter, Lady Charlotte.

Stephen met Lydia in St. Petersburg as he came into his inheritance. "Along with the title," Follett writes in a flashback to 1895, Stephen got, "several thousands of acres in the south of England, a big chunk of Scotland, six race horses, Walden Hall, a villa in Monte Carlo, a shooting box in Scotland and a seat in the House of Lords."

He contracted a loveless marriage with Lydia--a highly strung, beautiful girl. Stephen needed a wife. Lydia married him for a mysterious reason buried in her past.

Ninteen years later, Stephen has fallen in love with his wife. They are living out the last days of the idyll of the English rich. Their daughter, Charlotte, is about to make her debut. Stephen, who takes his conservative politics seriously, knows that the world is coming unraveled. So does Asquith's government. It dispatches young Winston Churchill to persuade Stephen to become the negotiator in an attempt to secure an alliance with Czar Nicholas II.

Lydia's dashing young cousin, Prince Aleksy Orlov, arrives in London to begin secret bargaining with Stephen. He is followed by a tormented anarchist, Feliks Kschessinsky, who is determined to kill Prince Aleks, enrage the czar and keep Russia out of the coming war.

Feliks is the man from St. Petersburg, the shatterer of comfortable assumptions. He is also the reason why Lydia married Stephen in 1895. She had had a passionate love affair with Feliks. Her wretched father had the young anarchist arrested and tortured. Lydia was told that her lover's torment would stop if she married and went away.

All of this--Feliks' stalking of Aleks, Scotland Yard's pursuit of Feliks, the rekindling of Lydia's buried passion, 18-year-old Charlotte's awakening to a world of stunning political and sexual realities--would seem contrived if Follett didn't write so beautifully.

The Man from St. Petersburg is a novel of atmospheres and characters in which the conventional threat-and- chase plot is secondary. Follett evokes a world of masters, servants, kings and suffragettes, Hogarthian cockneys and Irish immigrants, of London's unique juxtaposition of privilege's light and poverty's squalid darkness.

And yet plot remains, and is damaged by Ken Follett's very absorption in his characters. He suspends any moral viewpoint as the novel's focus shifts from Feliks to Lydia to Charlotte to Stephen and back to Feliks again. Follett creates a surfeit of sympathies. We understand all these people completely. Through our understanding, all of them seem justified (except for Lydia who has one hystrical fit after another and doses herself with laudanum in between). Therefore, The Man from St. Petersburg lacks the tension which comes from the true thriller's art of presenting us with heroes and villains--however complex and flawed their heroism and villainy may be.

And yet the evocation of an era, the replay of lives now lost and the power of attitudes now abandoned in a world without honor, makes Ken Follett's fourth novel an endearing and absorbing story.

Endearment, like honor, is in woefully short supply just now.