Professor Stangerson has been working and living for fifteen years with his daughter in their castle, Glantier. A few days after Miss Stangerson decides to marry Robert Darzac, who has been courting her for fifteen years, somebody tries to kill her. A young reporter, Rouletabille, will try to solve the mystery. The yellow room where the attack took place was closed from the inside, so how did the attacker get there? During the research he finds out the killing was not the only purpose of the attack. Mr. Stangerson's scientific research disappeared.
In the meantime, there is another official inquiry lead by inspector Larsan who suspects Darzac. But Rouletabille thinks that Larsan's behavior is strange. He uses a cane that he pretends to have from London, but the reporter knows it was bought from Paris by a man who looked just like Darzac. A second attempt to kill Miss Stangerson takes place and this time a man gets killed by father Jacques: the guard known as “the green man”. But when they examine the body they realize that he was hurt before Jacques hit him. Darzac is arrested but Rouletabille promises to prove his innocence. Rouletabille says that he knows the name of the assassin but will not tell it until a certain hour.
At 18:30 he says the name of the murderer: F. Larsan, whose name was actually Ballmeyer. He was a French con man who fled to America, took the identity of Jean Roussel and met the young Mathile Stangerson, who lived with her father in America. They fell in love but Dr. Stagerson did not agree so she sent his daughter to Ohio. Roussel came after her but he was arrested his real name, Ballmeyer, was revealed to the girl.
This report prepared by Dana Samson