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...