He flew to Great Britain in May 1941 -- parachuting from his Messerschmitt Bf 110 into Ayrshire on May 10 -- in what he thought was a secret mission to negotiate peace with the Duke of Hamilton. He was immediately imprisoned by the British (in the Tower of London), and his attempt was dismissed by Hitler. This journey was one of the odder events of World War II, and was actually a scheme conceived by James Bond author Ian Fleming (source The Man Who Was M - ISBN 0-631-13392-5), who was an officer in British Intelligence at the time.
The trap was laid in 1940? after Fleming read about the Anglo-German organisation The Link in the intelligence file of it's founder, Admiral Sir Barry Domvile. Via an agent Fleming fed Hess the line that The Link had been driven underground and was in a position to overthrow Prime Minister Winston Churchill and negotiate peace, and that the Duke of Hamilton was prepared to be a negotiator. Hess selected the date of his flight after Ernst Schulter-Strathaus, Hess's consultant on astrology and the occult, told Hess that there would be an alignment of 6 planets in Taurus at the time of the full moon on May 10.
Hess was tried at the Nuremberg Trials after the war for crimes against peace and was given a life sentence. For decades afterward, he was addressed simply as "prisoner number seven". Following the 1966 release of Baldur von Schirach and Albert Speer, he was the sole remaining prisoner of Spandau Prison. He degenerated mentally and apparently lost most of his memory.
In 1987, he died under Four Power imprisonment in West Berlin - kept in prison at the insistence of the Soviet Union which apparently had never forgiven him. His death was ruled a suicide. His son Wolf Rüdiger Hess maintains to this day that he was murdered by the British SAS. Eugene Bird wrote a novel about him titled The Loneliest Man in the World.