With sales of her books topping 4 billion copies worldwide, this much is clear: Agatha Christie (1890-1976) knew how to spin a mystery. Now it turns out she knew how to live one, too.

In a plot twist fit for one of her delectable tales, Christie secretly stashed a finished short story amid the notebooks and journals in which she sketched out her books.

The almost 6,800-word story, never published in her lifetime, features Hercule Poirot, the dapper detective made famous in several of Christie's novels.

After Christie's daughter, Rosalind Margaret Hicks, died in 2004, the story was discovered in her mother's papers, stored in Hicks' attic.

Titled "The Incident of the Dog's Ball," it was published in England three months ago.

Its first American publication is in this month's installment of The Strand (strandmag.com), the mystery magazine with editorial offices in Birmingham, Mich. Now in its 10th year, The Strand is a rejuvenated version of the legendary British periodical of the same name that, before its demise in 1950 after more than a half-century of operation, routinely published such renowned authors as Sir Arthur Conan Doyle, W. Somerset Maugham -- and Christie.

"Agatha Christie published dozens of stories in the old Strand, which is why we're so excited," says Andrew F. Gulli, The Strand's managing editor.

He calls the newly discovered story "a vintage Agatha Christie whodunit."

Gulli has made a specialty of such finds: In earlier issues, he published heretofore unknown works by Mark Twain and Graham Greene.