Paris, 1899. President Félix Faure has just died in the arms of his mistress, Marguerite Steinheil (Meg). With the Republic on the verge of explosion, Prefect Lépine is called back to business.

The courtesan Meg, who became a spy on behalf of Puybaraud, head of the police and sworn enemy of the Prefect, infiltrates the Guérins.