In Paris, in the 1920s, inside a dusty attic of the Parisian PJ, a police laboratory unique in the world took shape. Julien Valor, a tortured but visionary police officer, is in command of this very special unit. He is accompanied by Léa Perlova, an independent and feminist scientific expert, Pierre Cassini, an old-fashioned field cop, Marius Delcourt, a police officer gifted for inventions of all kinds, Martello, a young idealistic cop, and Pauline Kernel, a young bourgeois woman who is taking her first steps as a forensic doctor. With the will of the pioneers, these men and women fight tirelessly to impose their new methods... They try to perpetuate this unity whose existence is regularly called into question by the traditional police...