The Mystery of the Yellow Room (French: Le mystère de la chambre jaune) is a mystery novel written by French author Gaston Leroux. Plot summary Reporter and amateur sleuth Joseph Rouletabille is sent to investigate a criminal case at the Château du Glandier and takes along his friend, the lawyer Jean 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 give no useful testimony. Rouletabille meets and interrogates several characters: the castle's concierges, Mr and 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, Frédéric Larsan, who has been assigned the case. Larsan suspects Mlle. Stangerson's fiancé, another scientist called Robert Darzac, to Rouletabille's dismay. More attempts are made on Mlle. Stangerson's life despite Rouletabille and Larsan's protection, and the perpetrator appears to vanish on two occasions when they are closing in on him, echoing Professor Stangerson's research into "matter dissociation". The gamekeeper is murdered during the second attempt. Ultimately, Larsan arrests Darzac who is charged with murder attempts. Rouletabille suspects that Darzac has secret reasons not to defend himself and he disappears to make further investigations. Two-and-a-half months later, as Darzac's trial opens, Rouletabille reappears sensationally and tells the court that the culprit is Frédéric Larsan himself, whom he accuses of being an alter-ego of a master criminal called Ballmeyer. Larsan appeared to vanish on the two occasions he was nearly collared as he was one of the pursuers. Darzac is released when it emerges that Larsan has vanished after Rouletabille warned him he would accuse him in court.