Hans Gisevius was born in Germany in 1904. After studying law he was employed by the Prussian Ministry of the Interior. When Adolf Hitler came to power in 1933, he became increasingly concerned about the illegal activities of the police. After the Night of the Long Knives he left the ministry and went to work in the private sector.
Gisevius had compiled dossiers on Nazi officials which he eventually took to Wilhelm Canaris and Hans Oster of Abwehr. In 1939, Canaris employed Gisevius as chief of special projects. The following year he was sent as military intelligence officer in the German consulate in Zurich where he made contact with Allen Dulles of the Office of Strategic Services.
With Wilhelm Canaris and Hans Oster, Gisevius began to plot the overthrow of Adolf Hitler. When Gisevius was exposed as being involved in the July Plot in 1944 he fled to Britain. Hans Gisevius, who provided important evidence against the Nazi leaders during the Nuremberg War Trials, died in 1974.
Sources: Spartacus Educational, Wikipedia