Countess Isabelle de Maunbassant, an old acquaintance of Lupin, has invited him to catch the murderer of her husband Philippe, offering him her husband's emerald ring as a reward. She invited the three suspects to her chateau in order to set a trap for the culprit: gem collector Frederick Autorett, the count's nephew Allan Dubois and failed businessman Pierre Schmidt. However, in the midst of her announcement, a blackout hits the chateau and the Countess is fatally shot. Just before she dies, she seals off the chateau, giving Lupin time until dawn to find the murderer. Posing as Jim Barnett, a private eye, and assisted by Isabelle's maid Marie, Lupin quickly finds and exposes the culprits: Allan Dubois and his twin brother, who secretly entered the chateau and committed the murder. With the case solved, Lupin respectfully leaves the ring with the Countess and invites Marie to join him in a prospective career change as a private investigator.