Reporter and amateur sleuth Joseph Rouletabille is sent to investigate a criminal case at the Chateau du Glandier and takes along his friend the lawyer Sainclair, who narrates. Mathilde Stangerson, the 30-something daughter of the castle's owner, Professor Joseph Stangerson, was found near-critically battered in a room adjacent to his laboratory on the castle grounds, with the door still locked from the inside. She recovers slowly but can make no useful testimony. Rouletabille meets and interrogates several characters: the castle's concierges Mr & Mrs Bernier, the old servant Jacques, an unfriendly inn landlord and a womanising gamekeeper, and begins a friendly rivalry with France's top police detective Frederic Larsan, who has been assigned the case. Larsan suspects Ms Stangerson's fiance, another scientist called Robert Darzac, to Rouletabille's dismay."