A goods train speeds through the Sapperton Tunnel, but disaster strikes before it can reach the other end as it collides with an unusual blockage on the line: seven sheep penned onto the tracks. Specially requested to investigate the carnage in Gloucestershire, Inspector Colbeck and Sergeant Leeming are confronted with a bizarre case unlike anything they've encountered before.
Stephen Rydall, a director of the Great Western Railway, is convinced that the tragedy is a personal attack on him and fears for the safety of his shepherd, missing since the incident. Rydall has enemies: a churlish squire, the scheming principal of the Royal agricultural College and a local villain, always one step ahead of the law. But, as Colbeck discovers, there's a prime suspect much closer to Rydall's own home.