"Bad things happen in threes."
This is a modernization of the phrase "three on a match," a superstition dating from the Crimean War that was popularized during the First World War. It was purported that if three soldiers shared a match for their cigarettes, the length of time it would take for all three to be lit was long enough for an enemy sniper to pinpoint their shared location. The flame from the first man sealed the fates of the second and third who were illuminated by the light.
A few years after the end of the Great War, two strangers arrive in Heathbury, a small village in southern England, on the 10:28 train. Seth Marlowe is a retired hunting guide from Kenya, but even he can't predict the movements and motives of the beautiful, but secretive, young woman sitting across from him. Both are invited to the annual spring fete thrown by one of Heathbury's landed families.
All the inhabitants of Heathbury whisper this foreboding saying after the death of the mysterious, glittering socialite is followed by the murder of another village resident. Newspaper heiress Theodora Raffington has taken it upon herself to report on the murders for the local paper, despite the warnings of her stoic lifelong friend, Constable Thomas Spode, and her fiancé, Lord Edmund Cheltenham. By nosing into the secret life of the victim, she inches closer to becoming the third casualty of a hidden marksman.