During the celebration of her Champagne house's centenary, Mrs. Dupaquier is murdered in her own cellars. The investigation is entrusted to the energetic Captain Sofia Guerini and the more reserved Gendarme Mathieu Delorme, a duo who have grown fond of each other after working together on several cases. Faced with the maze of tunnels connecting the cellars and the mysterious inscription left by the murderer, they must call on Bastien Keller, a geologist who specializes in the region. For the purposes of the investigation, Keller is exceptionally released from prison, where he is being held for violating his probation—he is under investigation for endangering his employees during underground work. One of the victims is still in a coma: Lucie, Sofia's fiancée. As a motive for revenge emerges, Sofia and Keller must put aside their differences to prevent further murders.