The world of unknowable objects - magical items that most people have no idea possess powers - has been quiet for decades. But three current members of a secret society have remained watchful, meeting every six months in the basement of a bookshop in London. They are pledged to protect their archive of magical items hidden away, safe from the outside world - and keep the world safe from them. But when Frank Simpson, the longest-standing member of the Society of Unknowable Objects, hears of a new artefact coming to light in Hong Kong, he sends the Society's newest member, author Magda Sparks, to investigate. Within hours of arriving in Hong Kong, Magda is facing death and danger, confronted by a professional killer who seems to know all about unknowable objects, specifically one that was stolen from him a decade before.