A retired Sherlock Holmes, now in his seventies and disguised as a Cambridge professor, is dramatically disturbed one day when a modestly successful author in his late-sixties named Arthur Conan Doyle comes to call. This Conan Doyle, notable only for his historical romances, science fiction, and a three-volume history of the Boer War (but no detective tales), knows somehow of the false professor's true identity. To Holmes's surprise, he asks for help in solving a case that strains the limits of credulity by suggesting the existence of parallel worlds.
Sherlock Holmes on one last adventure in 1920s' London...employed by the author Arthur Conan Doyle? We might ask ourselves how such a thing can be possible.
This tale is discovered in a long-forgotten manuscript in 1940s' Buenos Aires. The librarian who finds it employs an Argentinian P.I. to help him grasp the deepest implications of the text. The story of the two Argentinians' brief and ultimately violent encounter provides a hardboiled frame to the last endeavor of Sherlock Holmes.