Antoine Rossignol worked to maintain the power of Louis XIV. He developed the most advanced coding system of the time.
It will also take the soldier and cryptologist Étienne Bazeries three years to fully decipher it, at the end of the 19th century. The Grand Siècle unfolds, from the arrest of the ambitious Fouquet to the conspiracy of Latréaumont, seen through the peephole of the king's dark cabinet. Interviews with historians (Jean-Christophe Petitfils, Flavie Leroux...) alternate with honest scenes of reconstruction in wig and costume. Francis Perrin plays Rossignol – a curious career than that of the hero of the film.