At a Prisoner of War camp for Germans in the north of Scotland, Kapitän zur See Willi Schlüter – a submariner – challenges the authority of the camp’s embattled Commanding Officer, Major Perry. British Army Captain Jack Connor arrives to investigate what's happening at the McKenzie POW Camp. Connor believes the camp disturbances are a cover for an escape attempt. During a mass brawl two POWs escape dressed as British soldiers and Connor notices an outcast German POW named Neuchl, being dragged from the barracks and fleeing from the Germans. He is badly beaten and later that night in the hospital is strangled before Connor gets the chance to learn about Schlüter's plans. With Connor investigating the camp, Schlüter leads his 28-man escape party out of a tunnel the next day. Meeting the two escapees who have arranged a U-boat to pick them up, they all head for the coast. Unknown to Schlüter, Connor has broken the code used in letters sent by POWs to Germany and knows the plan. Connor, along with General Kerr, starts searching for the prisoners. The Germans head for the coast and burn their escape lorry, which is seen by a reconnaissance plane. Drawn by the burning lorry, Connor locates the Germans attempting to paddle towards a surfaced U-boat at dusk. Connor calls in a Royal Navymotor torpedo boat with depth charges to engage the U-boat. With only 50 yards to go, Connor orders the pilot to 'buzz' the inflatable dinghies, delaying Schlüter's craft, and with the MTB arriving, the U-boat dives, leaving Schlüter and three comrades stranded.
The film was based on the novelThe Bowmanville Break by Sidney Shelley. Film rights were bought in January 1968, prior to the novel's publication, by the producing team of Jules Gardner, Arthur Levy and Arnold Laven, who ran LGL Productions and had a deal with United Artists. William Norton, who had done several scripts for LGL, was assigned to write the screenplay. The location of the story was shifted from Canada to Scotland. In October 1968 Brian Keith signed to play the lead role. That month the novel was published. The New York Times called it "a crackling tale". "In the best tradition of escape literature," said the Chicago Tribune. In February 1969Andre De Toth was signed to direct. By April he had left the project and been replaced by Lamont Johnson. The film had been retitled The MacKenzie Break. The film was shot in Ireland, at Ardmore studios Co. Wicklow, and in Bonmahon Co. Waterford, in October 1969.