Fed up with his desk duty in the Imperial Arcane Library, book hunter Colin Bliss accepts a private commission to find The Sword's Shadow, a legendary and dangerous witches' grimoire. But to find the book, Colin must travel to the remote Western Isles and solve a centuries' old murder.

It should be nothing more than an academic exercise, so why is dour — and unreasonably sexy — Magister Septimus Marx doing his best to keep Colin from accepting this mission — even going so far as to seduce Colin on their train journey north?

Septimus is not the only problem. Who is the strange fairy woman that keeps appearing at inconvenient times? And who is working behind the scenes with the sinister adventuress Irania Briggs? And why do Colin's employers at the Museum of the Literary Occult keep accusing Colin of betraying them?

As Colin digs deeper and deeper into the Long Island's mysterious past, he begins to understand why Septimus is willing to stop him at any price — but by then, it's too late to turn back.