Avenging Joan of Arc and Trafalgar, Arsène Lupin robs the Tower of London. Fabulous booty: the Crown's treasure and, above all, a priceless parchment, a thousand-year-old relic shrouded in mystery, the secret of William the Conqueror. Herlock Sholmes is assisted in his investigation by the Intelligence Service, the French 2nd Bureau, Commissaire Guerchard and journalist Isidore Beautrellet. A scholar tells them that a copy of the famous parchment, of which he has transcribed indecipherable passages, was stolen from a monastery in Normandy five years ago...