The story is essentially a first-person narrative by Nicholas Slopen, an academic who specialises in the writing of Samuel Johnson. He is asked to authenticate some newly discovered Johnson letters. Their style and content seem to be genuine, but when Slopen sees the original manuscripts he realises at once that they are forgeries.
This is the beginning of a series of progressively more sinister revelations. Slopen is now told that the letters have been written by a savant, an apparently autistic man who writes and speaks as if he were Samuel Johnson. But this is not the full truth either; later it turns out that the man is a vagabond who has been subjected to what Slopen calls the Procedure. His mind has been replaced with a synthesised version of Johnson's mind, constructed from Johnson's writings. The Procedure has been developed by a shadowy group of people based in Russia, to provide them with a form of immortality. This discovery ultimately has disastrous consequences for Slopen himself.
Considered as a thriller, the book isn't written in the style you might expect for the genre. It is complex and literary, with a fair sprinkling of unusual words. There is some comedy, especially when the pseudo-Johnson comes to stay with Slopen and struggles to understand the twenty-first century, but the tone is mostly sombre and melancholy.
I generally enjoy books of this kind but I have to say that this one didn't entirely come off for me. In part this may have been because of a couple of medical errors. When an unknown man is brought by ambulance to hospital and dies en route, a hospital doctor writes a certificate saying death was due to cardiac arrest. This wouldn't happen; the case would certainly need to be referred to the Coroner. Later, someone shines a light into Slopen's eye 'to dilate the pupil'; it would, of course, constrict it.
These are minor blemishes, but I found a more serious problem with the plausibility of the Procedure. There has been a lot of speculation, fictional and non-fictional, about the possibility of uploading a mind, conceived of as software, from a brain into a computer. In fact, this is practically a cliché today. The Procedure (which is not described even in outline) would be another version of this scenario, but using written information rather than analysis of a subject's brain activity. I found it hard to believe that anyone's writing, no matter how voluminous, could provide enough material to reconstitute a whole personality and consciousness.
Slopen has an agreeably acerbic wit and his character comes across quite well. But I found the metaphysical basis of the story to be less intriguing than I had hoped. For two other novels that I think work better in this respect, see The Victorian Chaise-Longue, by Marghanita Laski, and Breakthrough, by Ken Grimwood. Both these books describe people who are trapped in alien bodies and both are genuinely terrifying, which Theroux's book wasn't at least for me.