When Hannah is invited into the first-class carriage of the London to Penzance train, she walks into a spider's web. Now a poor young single mother, she once escaped Cornwall to go to university, but after marriage to Jake her dreams turned to bitter disillusion. Her husband has left her for a rich woman, and Hannah has survived by working as a cleaner. Jinni is equally angry, and in the course of their journey the two women agree to murder each other's husbands. After all, they are strangers on a train - who could possibly connect them?
But when Hannah goes to Jinni's house she meets its shambolic caretaker, who claims Jinni is very different to the person Hannah had been led to believe. Who is telling the truth - and what will become of the women's pact to commit a terrible crime?