The Mystery of the Yellow Room

At midnight, Mathilde is attacked while alone inside the locked Yellow Room. The room is found with Mathilde unconscious, two bullets lodged in the ceiling, and a bloody handprint on the wall. The intruder escapes, triggering a perilous mystery the investigators must unravel.

Rouletabille and his photographer Sinclair travel to the castle to solve the mystery. On the train they meet Judge De Marquet and his aide, who are also there to investigate, adding official weight to the case. The journey sets the stage for a complex, communal investigation.

At the castle they learn that the renowned detective Frederic Larsan is also present to help solve the case. The group begins listing possible suspects, including Mathilde's fiancé, the concierge and his wife, and the groundskeeper. A tense atmosphere settles as everyone becomes a potential link in a long chain of motives.

They uncover that Mathilde had called off her engagement two days earlier and that a coded letter upset her and her fiancé the day before. Rouletabille hints that something significant happened to Mathilde, suggesting hidden motives behind the engagement turmoil. The mystery deepens as relationships and secrets surface.

During dinner, Mathilde is observed spiking her father’s tea with a sleeping drug, enabling a later meeting with her attacker. The investigators realize Mathilde may be arranging a clandestine rendezvous that night. The incident underscores a planned, calculated approach to the crime.

Sinclair sets up a stakeout outside Mathilde's bedroom after Rouletabille and Larsan also fall under the sleeping drug at dinner. The wait is filled with tension as everyone hopes to catch movement from the room. The stakeout marks the first concrete attempt to observe the attacker’s patterns.

During the night, a figure wearing a welding mask slips into Mathilde's bedroom. Sinclair wakes Rouletabille and he uses a ladder to spy through the window, witnessing Mathilde pleading with the intruder and then kissing him passionately. The scene confirms a complex relationship driving the events.

A trap is set with Larsan and Father Jacques watching different exits, but the attacker vanishes again, eluding capture. The team regroups, realizing the perpetrator is highly elusive and strategic. The mystery tightens as days pass without a final breakthrough.

The attacker is found dead in the courtyard after his escape, and the groundskeeper becomes the initial suspect. Mathilde’s fiancé reappears the next morning and is arrested by Larsan, adding political weight to the investigation. The group wrestles with a rapidly shifting set of accusations.

Rouletabille informs Sinclair that he will leave to gather evidence and hints that he knows the killer’s true identity. He asserts that the groundskeeper is not the culprit, signaling a forthcoming revelation. The detective’s departure marks a strategic turning point in the case.

Weeks later, during the reconstitution, Larsan claims the groundskeeper was killed by the real assassin who fled while everyone was distracted. Rouletabille disagrees, insisting the truth lies elsewhere. The debate deepens as more pieces are put on the table.

At 6:30 pm, Rouletabille reveals that Larsan is actually Ballmeyer, a world-class acrobat thief in disguise. He explains the elaborate scheme, including Mathilde's secret marriage to Ballmeyer in America and the coded letter that spurred the events. Rouletabille notes he warned Larsan during the afternoon break, giving him a head start before the police arrived, and the fiancé is exonerated as the truth comes to light.