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.