When a homemade bomb explodes on a houseboat moored on the Seine, following two brutal stabbings in Pigalle, the Paris police receive a list of targets for assassination that includes the President of the Republic himself. Each murder will be announced beforehand by the arrival of a playing card.

Inspector Jean-Paul Gautier believes the choice of cards, their suit and value, are significant in some way, and must follow the threads of a riddle that unfolds against the colourful backdrop of life in Belle Époque Paris and climaxes in a spectacular bal masqué.