In Birmingham city, Railway Police Constable Charles Baron (John Horsley) is involved in a confrontation with a man believed to be a local vagrant. The man gets away, but he is soon found out to have been a saboteur, who has left a suitcase full of detonators and bomb-making components at the railway yard. Police realize that the man was attempting to sabotage a trainload of sea mines, destined for the Royal Navy Yard at Portsmouth. The train is stopped as soon as possible in case an explosion is imminent, but a residential area is nearby and the police have to evacuate local residents.