All across London, single mothers are vanishing. Margaret Hill, mother of two, walked out of her house two months before, never seen again. A month later, Carrie-Anne Morgans takes her two-year-old son for a walk in the park and disappears leaving him alone in his stroller. Lorna McCauley leaves her London flat in the early hours of the morning to buy medicine for her sick child and disappears. Newly promoted Detective Inspector Theophilus Blackwell is assigned the case of Lorna McCauley, which, on the outside seems to be a simple case of mid-life crisis and child abandonment. Elsewhere in London, MI5 analyst, Sophia Evans, is working undercover to catch an animal rights group responsible for targeted bombings. As her case (and her personal life) fall to pieces, she receives a strange envelope in the mail. It contains a picture of Lorna McCauley s lifeless face along with a daunting code. Now the police and MI5 are forced to work together to stop the murders, and Sophia must find her way into the terrifying mind of a serial killer.

An elderly man with only one leg is murdered and left in a pool of his own blood outside his house. To add to the mystery, a note found in his pocket says, 'Why Run Backwards You'll Vomit.' London Detective Chief Inspector Theo Blackwell can't understand the motive for killing the old man, or the meaning of the cryptic message. Later, a woman is stabbed on her doorstep. The two seemingly unrelated cases have two things in common: apparently random victims and suspects with alibis. As DCI Blackwell works on solving the cases, he requests the help of code-breaker Sophia Evans, who is battling a personal and tricky case of her own.