Warning: Contains significant plot spoilers
When a North Oxford housewife, Rachel Mallory, is found hanged at home, it appears to be an open and shut case of suicide. She leaves devastated husband Hugh, and two young daughters, 8-year-old Anna, and 9-year-old Izzie. Hugh's business partner, David Hayward, and his wife Louise, were close friends of Rachel and Hugh. With two boys of their own, they're concerned that the girls are protected from the trauma of their mother's death.
Much to the chagrin of CSI Innocent, Inspector Lewis' instincts tell him something's wrong about the suicide verdict. But whether it's the lack of a note, or the fact that Rachel's life seemed perfect, he's not yet sure.
Lewis receives a request for a meeting from a highly respected elderly Oxford law professor, Edward Le Plassiter. Lewis and Hathaway discover that Le Plassiter is dying and wants to make his peace with life. To this end, he wants to strike a deal with Lewis. If the detectives will track down a former student — Stoker — whom Le Plassiter gravely wronged years ago, the old man will in turn provide insight into the death of Rachel Mallory. With the professor at death's door, and no other leads with the Mallory case, Lewis has little to lose in going along with this. To prove he can deliver on his side of the bargain, the professor provides Hathaway with a name: Jane Templeton. When he and Lewis arrive at the woman's home, they find her dead — another apparent suicide case.
Lewis and Hathaway face a barrage of impenetrable questions and come up with few answers. What's the link between the Templeton woman, and Rachel Mallory? If Mallory's death was in fact not a suicide, then who had any motive to murder her?
Lewis strikes up a burgeoning friendship with the Mallory's single-mother neighbor, Stephanie Fielding, who points him in the direction of a key piece of information: five years ago, when the Mallorys and the Haywards went on holiday together, a very peculiar exchange took place — something which testified to the particular strength of their friendship.
Dr Hobson, who was away at the start of the case, reverses the verdict on Rachel Mallory's death. It was, in fact, murder disguised as suicide. The same applies to Jane Templeton.
With everything pointing to a single killer, but David and Louise Hayward and Hugh Mallory providing no help and no discernible motive, Lewis returns to the increasingly frail Le Plassiter. Moments before losing consciousness, the old professor reveals that he used to advise the Home Office on certain delicate legal matters — including the best ways to handle child murders. He leaves Lewis and Hathaway with just enough to go on to pinpoint their killer, but they face a heart-stopping race against time to try and avoid three more tragic deaths.