In early 1939, before the start of the Second World War, Sir Robert Hunter (O'Toole) takes aim at Adolf Hitler with a hunting rifle, but hesitates to shoot and is spotted and tackled by a Schutzstaffel (SS) guard. Captured and tortured by the Gestapo, he claims that his aiming at Hitler was simply an intellectual exercise, to see if it could be done. Because of his high status in Britain, his captors intend to shoot him and disguise it as a hunting accident, but, because his body displays clear evidence of torture, they decide instead to throw him off of a cliff to disguise the signs.

However, Hunter survives the fall, and, with the aid of a German fisherman and a friendly sailor, makes his way back to England. He soon discovers that German agents are after him, and that the Nazi government has requested his extradition. After killing a pursuer by electrocuting him on the London Underground, Sir Robert, now a fugitive, goes underground in rural England to escape his pursuers. It is revealed through flashbacks that his girlfriend, who played a role in organizing the German Resistance, was executed by the Nazi government.

Hunter's lead pursuer, an English Nazi sympathizer named Major Quive-Smith, successfully tracks him to his lair and traps him within it. Rather than kill Hunter, Quive-Smith offers to spare his life if he signs a false confession stating that he attempted to assassinate Hitler on behalf of British intelligence. Over the course of several days, Hunter constructs a rudimentary crossbow, lures Quive-Smith into looking down his breathing hole, and shoots him in the head, killing him.

Soon afterwards, Britain declares war against Germany, and Hunter, now with his status restored, agrees to see the Admiralty about a secret mission (heavily implied to be a second attempt on Hitler's life). (fonte: Wikipedia)