Sherlock Holmes, now in his seventies, retired from investigations and peaceably disguised as a professor at Cambridge, is shaken when a modestly successful author in his late-sixties named Arthur Conan Doyle calls upon him at the university. This Conan Doyle, notable for historical adventure stories, science fiction, and a three-volume history of the Boer War (but no detective tales), somehow knows of the false professor's true identity and pleads for investigative assistance. Someone is trying to kill Conan Doyle. Who? Why? Good questions, but what intrigues Holmes most is how the "middling scribbler" ascertained Holmes's identity in the first place, despite the detective's perfect disguise. Holmes takes the case.
There is danger every step of the way. Great powers want the investigation quashed. But with the assistance of Dr. Watson's widow, Holmes persists, exploring seances, the esoterica of Edgar Allan Poe, the revolutionary new science of quantum mechanics, and his own long-denied sense of loss and solitude.
Ultimately, even Sherlock Holmes is unprepared for what the evidence suggests.