A student in the Reading Room of the London Library, Marion Summers, suddenly goes into a seizure, then slips into a coma and then dies. But what first appears to be a simple tragedy, is soon revealed to be much more sinister. During the autopsy, it's determined that she actually died of acute arsenic poisoning. Brought to the attention of the Serious Crime Unit of Scotland Yard, the newly promoted Detective Inspector Kathy Kolla, with the help of her superior, DCI David Brock, investigate the mysterious Summers, leading them to suspect her death was related to unexplained and unusual aspects of her recent life. The more they investigate, the more confounding the mystery becomes and more the clear it is that behind what really happened - and why - lies the most difficult-to-crack case Kolla and Brock have ever faced.