In 1864, banker Thomas Briggs is beaten and thrown from a first-class carriage on a North London Railway train, the first murder ever committed aboard a moving British train. The film reconstructs how the crime upended Victorian confidence in rail travel as a safe, modern convenience, and follows Scotland Yard Detective Inspector Richard Tanner as he pieces together the case from a bloodied gold watch and a crumpled hat left behind in the carriage. The trail leads to suspect Franz Müller, who has already boarded a ship bound for New York, triggering a transatlantic pursuit and one of the era's earliest international extradition cases. Historians walk through the forensic evidence, the public panic that followed the murder, and the courtroom arguments that set precedent for how Britain would handle criminals fleeing abroad. The case ends at the gallows, but the film's real interest is in what it exposed: a public suddenly aware that the railway carriage, symbol of industrial progress, could also be a locked box with a killer inside.