It's a race against the clock to defuse a time bomb on board a train filled with high explosives.

As a freight train loaded with explosives passes the depot in Birmingham, England, a man who was hiding onboard jumps off and, after fighting off a railway security officer, Constable Charles Baron, runs away. At the office of the railway police, Sgt. Collins examines the satchel the man left behind when he fled, and discovers wires and a box of detonators. Upon learning that the train contained sea mines headed for the naval shipyard at Portsmouth, Collins notifies zone security official Jim Warrilow, who immediately summons Sir Evelyn Jordan, the chief constable of Birmingham. The police search without success for the fugitive, and make plans to divert the train, out of fear that its cargo may be rigged to explode. On the theory that the saboteur may intend to witness the explosion, Baron is dispatched to Portsmouth to look for him. The police then decide to contact Major Peter Lyncort, a former member of the Royal Canadian Engineers' bomb disposal unit who now lives in Birmingham. Meanwhile, Peter is home with his French wife Janine, who complains that their marriage has grown dull since the end of the war. When Peter refuses to leave his job and accompany her to Paris, Janine sadly walks out. Meanwhile, the train is diverted to the town of Felsworth, where it is stopped and ordered to abandon its load. After the cars are disengaged and left on the tracks, the town's residents are evacuated. Warrilow goes to Peter's home and informs him of the sabotage, adding that the train was due to arrive in Portsmouth in less than eight hours. Peter accompanies Warrilow to Felsworth and, using an electric stethoscope, examines each mine on the train. Warrilow insists on helping with the search, and Peter reluctantly gives him a quick lesson in bomb detection. Meanwhile, Janine has second thoughts about leaving Peter, and a policeman at the train station tells her about the trouble down the line. After calling home and getting no reply, Janine waits for Peter at their favorite restaurant until closing time. As she prepares to leave, the proprietor tells her that news reports say someone is working to find and disarm the bomb. Janine returns home and grows melancholy when she finds Peter gone. As dawn breaks, Peter and Warrilow still have eleven cars to inspect in only two hours. Meanwhile, at the Portsmouth train station, Baron at last spots the saboteur and captures him. Janine grows worried and starts calling the hospitals, and learns that Peter came in the previous night to borrow an electric stethoscope. Finally realizing what Peter is doing, Janine drives to Felsworth in a panic. Meanwhile, Constable J. Reed brings Peter and Warrilow word that the saboteur has been caught and is being brought to Felsworth by naval helicopter. Warrilow finds a wire in one of the mines, and Peter orders him away while he examines it. Peter extracts a crude explosive device from inside the mine and nervously disarms it, then suggests to Warrilow that they not let the saboteur know that the bomb has been found. Janine arrives in Felsworth just as the police announce that the danger has been averted and people may return to their homes. Later, the saboteur is brought to the train, and Warrilow handcuffs him to one of the cars, saying he will be released when he provides information. The man grows increasingly nervous as time passes, and as seven o'clock approaches, cries out for help. Warrilow goes to question him, and is surprised to learn that the saboteur planted two charges in the same mine, and both are set to go off at seven. Warrilow quickly frees the man and informs Peter, who runs toward the train as the church clock begins to chime. Peter is reaching into the bomb when Janine runs up and refuses to leave his side. Peter removes the charge and hurls it from the train, and it explodes without igniting any of the mines. With their arms around each other, Peter and Janine walk away from the train.