One morning in May 1882, Inspector James Hadley arrived in his office at Scotland Yard and was immediately summoned by Chef Inspector Bell to a meeting with the Commissioner. In grave tones the Commissioner ordered an immediate inquiry into the hideous murder of an ex-army man living in Islington. Hadley and Bell were told that the Bishop of London had just received a letter from the victim informing him that a priceless religious relic had been discovered and requesting an urgent meeting. As Hadley and Sergeant Cooper begin the investigation, the victim's brother is found strangled in his lodgings in Whitechapel. More murders follow and the detectives are hard pressed to bring the killer to justice, when it is revealed that the relic is the Holy Grail. French detectives arrive in London to claim the Holy Grail for France, which complicates the investigation. More murders are committed as the trail leads to three castles in Scotland where Hadley, Cooper and the French Officers face death at the hands of two crazed killers' intent on finding the Holy Grail. Eventually one is overcome and the other arrested, but not before strange and unexpected twists in the investigation as it draws to a final conclusion.