At a military hospital near Dover in 1942, two convalescing officers, Maj. Freddie Oppenheimer, who lost a leg in combat, and Maj. Pierre Debrie, who lost a hand, judge themselves fit for active duty and devise a plan for demolishing a German gun which has been shelling the coast. Accompanied by the reluctant Lieutenant Pringle, an explosives expert with no tolerance for noise, and a veteran colonel, they leave the hospital and make their way across the English Channel in a stolen boat. Once in France, the four narrowly escape detection by German patrols and disguise themselves in stolen German uniforms. After dismantling some of the offending gun's mechanism, the invaders enlist the aid of the Germans to divert a trainload of ammunition to the gun site. Their target demolished, they make a slow escape on bicycles, but Pringle becomes separated from his comrades. As the others near the coast, members of the French underground mistake them for Germans and take them prisoner. However, with the reappearance of Pringle, whose atrocious French accent convinces the Maquis that he could only be English, the four men are permitted to return to Dover. Now hailed as heroes, they are informed by British Intelligence that another German big gun requires their immediate attention.
