In 1924, at just 20 years old, Otto Rahn earned his degree in history. A brilliant scholar, Rahn's intelligence was overshadowed only by his obsession with locating the Holy Grail. The Grail, supposedly the cup Christ used at the Last Supper, has long been debated among historians—many question whether it ever existed. But Rahn was certain it had survived into the 20th century, waiting to be found. Believing that lines from the medieval poem Parzival held clues to the location of the Grail, Rahn embarked on his first expedition to the former strongholds of the Cathars in Southern France, a now-extinct branch of Christianity. Although he didn’t find the Grail, he published a book detailing his search. This book caught the attention of Heinrich Himmler himself. Himmler, the fanatical leader of the SS, was as obsessed with the Grail as Rahn. Upon reading Rahn’s account, Himmler decided that this brilliant historian—despite his open opposition to Nazi ideology—might be the man to finally unearth the sacred relic...