In 1926 Cambridge history don and Restoration buff Nicholas Felton does a deal with the Devil to be allowed to go back in time to 1675, to take over the body and temporarily the life of his ancestor, Sir Nick Felton, a staunch Royalist but also, as Felton (to distinguish from Sir Nick) soon discovers, an ogre. Luckily, it's the more urbane Felton who's normally in charge of the proceedings as he swaggers and swordplays his way through the London of Charles II's time; occasionally, however, in times of great emotional stress (usually rage) the monstrous Sir Nick takes over and Felton blacks out, to be told afterwards of what havoc Sir Nick might have wrought.
Felton's ostensible purpose in coming back to 1675 is to try to solve the mystery of who killed Sir Nick's wife Lydia -- and soon, as he becomes completely infatuated, physically and otherwise, with Lydia, his more important self-appointed task is to prevent the murder happening at all. And at first it seems he has succeeded: he identifies the servant who has been feeding Lydia a slow, subcritical diet of arsenic, and the trick whereby it's being done. Yet he exhibits mercy toward the culprit, knowing she was but someone else's catspaw. Because of his decency in this unmerciful age, he soon becomes and object of devotion for his servants. They approve, too, of his ejecting from the household one Meg York, Sir Nick's unconcealed mistress this past while, and who bears a quite astonishing resemblance to demure Mary Grenville, the daughter of a friend of Felton's back in 1925.
There's lots of swashbucklery, and by book's end all the machinations of the plot to kill Lydia (and other plots, equally murderous) have been exposed and satisfactorily explained.
John Dickson Carr is perhaps my favourite of the classic detective writers, and so obviously I read this book decades ago. All I could remember of it were the vague general setup and that it had taken me quite a time before the novel started gripping me. Exactly the same happened this time. Carr evidently did huge amounts of research for this, his second historical novel (his first, The Bride of Newgate was a straightforward historical; two more timeslip fantasies followed The Devil in Velvet, and I shall be reading them shortly). That research shows, oh gawd does it show. Aside from frequent pauses (at least during the earlier part of the book) to offer minute descriptions of architectural features or niceties of attire, the characters all speak in a vocabulary that I'm sure is filled with lots of authentic flourishes but is a bit bloody boring to wade through. Still, once the author gets over the fact that he needs to impress us with his historical erudition, things start zipping along merrily enough, in true Carr style.
One interesting aspect of the book: Occasionally Felton, using his deep historical knowledge of the period, attempts to warn his new contemporaries -- including Charles II during an audience at Buck House -- of events that lie in their near future; of course, no one believes him, as otherwise history would be altered. Yet such alterations seem permissible in a small way. When Felton warns Charles of the Popish Plot,* that warning actually contributes to making the true plotters' duplicity yet more effective. It's a nice touch: you can't change history except to make it even more so, as it were.
* The Popish Plot was a wheeze dreamt up by unscrupulous politicians/courtiers, primarily Protestants, to cause civil turmoil through making up out of whole cloth a conspiracy by Catholics to overthrow the monarchy. That way they could cruelly persecute Catholics, get rid of a bunch of adversaries whose loyalty to the Catholic Charles was a nuisance, etc. Hello to the FOX News of the 17th century.