Commissioner Maigret rushes to the Quai d’Orsay with Lieutenant Janvier. There, they find Mademoiselle Larrieu in a state of shock. That very morning, she discovered the bullet-riddled body of Mr. Berthier-Lagès, a renowned former ambassador whom she had served as a housekeeper for 46 years. While searching the victim's apartment, Maigret and Janvier uncover hundreds of letters. The diplomat had maintained a fifty-year-long love correspondence with a certain Princess of Vuynes, whose husband - by a strange coincidence - had died just a few days earlier.
As he confronts the various members of the two families and the suspicious silence of Mademoiselle Larrieu, Maigret finds himself faced with surprise after surprise, watching his certainties crumble - until the solution to the mystery finally hits him in the face at the very last moment...