With London being battered in the Second World War by the awesome Nazi V2, Squadron Leader Quint (David McCallum) is handed the task of leading the elite Mosquito Squadron on a perilous mission to destroy a secret Nazi weapon site. The Germans are rumoured to be developing a new V3 weapon, but the target site, a French chateau, also contains hundreds of RAF POWs. It is decided that the mission must go ahead, regardless of the possible casualties.