From Wikipedia:
The Conspirators (original French title: Le chevalier d'Harmental) is a novel written by Alexandre Dumas and Auguste Maquet, published in 1843. Dumas reworked a preliminary version by Maquet; this was the beginning of their collaboration which was to produce eighteen novels and many plays.
The novel is set in 1718, and its subject is a conspiracy against the regency of Philippe d'Orléans, who led France during the infancy of Louis XV. Dumas and Maquet's later novel Une fille du régent features some of the same characters.
George Saintsbury described the novel as "usually ranked among [Dumas's] masterpieces"; Dumas himself felt it had both strengths and weaknesses. Richard Stowe, Dumas's biographer, called it the best of his novels set during the reign of Louis XV.