The 'French Sherlock Holmes', Inspector Girac, visits England in a desperate attempt to find someone trustworthy who can help him prevent a group of Jewish anarchists and their associate Jaques Huret, the Boulevard Assassin, from murdering the new President of the Republic in retaliation for Dreyfus's imprisonment. Girac harbours suspicions about his fellow comrades in the Surete but needs help in outguessing this dedicated assassin, which requires a genius. He calls upon Sherlock Holmes.