Disaster strikes at the Sapperton Tunnel in Gloucestershire when a goods train collides with an unusual blockage on the line: seven sheep penned onto the tracks. Specially requested to investigate the carnage, Inspector Colbeck and Sergeant Leeming are confronted with a bizarre case unlike anything they've encountered before.

Stephen Rydall, board member of the Great Western Railway that manages the route, is convinced this tragedy is a personal attack on him and fears for the safety of his shepherd, missing since the incident. Rydall has many enemies but, as Colbeck will soon discover, the man also has a closely guarded secret of his own...