February 1944, and Air Commodore Davies has just received one of Churchill's most feared ACTION THIS DAY directives. The Germans are shipping out a dangerous consignment of heavy water; once in Germany it will be processed in a bomb-proof installation. The threat is overwhelming; the stocks must be destroyed in transit. Davies is visibly shaken. It is an impossible mission -- yet the Air Commodore is hoping for a small miracle. Somehow the ace pilots of 633 Squadron must do the job, no matter the cost in material -- or the sacrifice of brace men's lives.