It reads like the plot outline of an Agatha Christie mystery:

The shy, reserved Englishwoman who writes detectives stories vanishes from her country house and family her motorcar is found precariously overhanging a highway bank; a fur coat discarded on the seat and an overnight case spiling out an evening dress, papers, and an expired driver's license; the missing-person bulletin mentions "no wedding ring"; police find two of three (or was it four?) letters left behind have been destroyed; newspapers post rewards and retouch photos with disguises of eyeglasses and rearranged hairstyles; a massive search of 15,000 with bloodhounds is mounted. Then, 11 days later, Agatha Mary Clarrisa Miller Christie reappeared as mysteriously as the lady vanished.

She turned up at a fashionable spa, registered under the name of her philandering husband's mistress.

Dame Agatha Christie, the dowager queen of mystery writers before her death 2 2/2 year ago, left one mystery unsolved - her own celebrated disappearance on Friday, Dec. 3, 1926.

If there is no Hercule Poirot or Jane Marple, to solve the baffling real-life Christie mystery, there is Kathleen Tynan, English journalist and novelist, to set up a set of similar circumstances and find a solution.

"Agatha" (subtitled "a novel of mystery") is Tynan's fictional investigation of the disappearance of a woman writer of detective fiction who is losing her husband to a younger woman. Was it foul play? Amnesia? Publicity stnt? An ingeniously-plotted revenge with a rail of red herrings" An escape into fantasy?

"This is not a who-dun-it. It is a why-do-it," Tynan explained this week on a promotion visit to Washington for her book, described as "an imaginary solution to an authentic mystery."

Tynan started out with theidea of reconstructing the 50-year-old mystery of the Christie disappearing act after reading Dame Agatha's obituary. It began as a televesion documentary, and then was transformed into a fictional screenplay, and then grew into a novel.

The film "Agatha" - with Dustin Hoffman as a reporter and Vanessa Redgrave as Agatha - will be released in December or early next year.

For legal reasons, both Tynan and her publisher (Ballantine Books) stress that "Agatha" is a fictional account of the disappearance, even if the names are the same. Dame Agatha, a very proper Englishwoman, thought publicity was vulgar and rarely granted an interview. Her famil has talked of legal redress for "merchandising" on Dame Agatha's name.

A book with Christie on the title page has always sold fabulously well. Also recently published has been another book. "The Mystery of Agatha Christie," by Gwen Robyns. It is subtitled "an intimate biography of the first lady of crime," and Robyns, who has done he legwork, offers her own speculative solution to the disappearance.

Both Tynan and Robyns retraced Agatha's footsteps. They talked to the daughter of the policeman in charge of the investigation, survivors in the nearby village of Sunningdale and retired staff of the Harrogate Hydropathic Hotel.

The spa exists. It was a wonderful Edwardian spa for London grandees," Tynan said. "The movie was filmed there and we knocked out a false ceiling to show the beautiful ironwork of the winter garden."

In real life, Mrs. Agatha Christie, the wife of Lt. Col. Archibald Christie, was reported to have joined the local orchestra for a song in the palm court and to have danced the Charleston.

She was registered at the Hydro spa as Mrs. Teresa Neele, only slightly changed from the name of Nancy Neele, her husband's secretary and paramour.

It was a band member of the Happy Hrdro Boys who tipped off police that the guest registered as Mrs. Neele might be the missing crime writer named Agatha Christie. On Dec. 14, 1926, she answered a reporter who addressed her as Mrs. Christie and that evening met her husband in the lounge. The family insisted the disappearance was a case of amnesia.

Tynan's fictional reconstruction is a psychological mystery - the state of mind that prompted a very proper Englishwoman of Victorian upbringing to disappear and become the center of attention in a sensational police case.

For Dame Agatha, in her own mysteries, the puzzle was the thing, not psychology. But she would have admired Tynan's tightly-tied plot and the stunner of a surprise twist at the end. It is worthy of Hercule Poirot.

"I think this is a very favorable portrait," Tynan said.

She remembers, word for word, a quote from Dame Agatha in one of her rare interviews, granted back in the 1960s: "My mother died a very painful death. My husband found a young woman. You can't change your fate. You fate comes to you. But you can do what you like with the characters you create."

In her autobiography, Dame Agatha discreetly avoids mention of her celebrated disappearance. There is a quote from Keats ("What shall I do to drive away/Remembrance from mine eyes?") and then she asks if one isn't entitled "to ignore those memories that one dislikes?" This is at the beginning of the chapter of the breakup of her first marriage.

Two years after Agatha's disappearance, the Christie were divorced, and Archibald Christie married Nancy Neele. In 1930, Agatha married archeologist Max Mallowan.

In Tynan's investigation, she talked to a psychiatrist about the Christie disappearance a half-century before the family story of loss of memory. She was told that most amnesiacs are "too busy trying to remember to invent persona" as Agatha did with her Mrs. Neele.

Tynan was a researcher for Newsweek magazine in New York in the early 1960s and worked for The Observer and the Sunday Times in London. She is married to Kenneth Tynan, the drama critic, theatrical producer ("O. Calcutta"), and former associate of Sir Laurence Oliver at the National Theatre in London.

She got her break to join the staff of The observer while doing research for Anthony Sampson, author of "The Anatomy of Britain" and a columnist for the newspaper.