They found Mathilde knocked unconscious on the floor, alone, two bullets lodged in the ceiling and a bloody handprint on the wall.
The reporter Joseph Rouletabille accompagnied by his friend and photographer, Sinclair arrive at the castle, eager to solve the mystery.
Working with each other, Rouletabille, De Marquet and Larsan, try to solve the mystery and prevent the attacker from coming back and finishing his undertaking.
Rouletabille also informs Sinclair that something important has happened to Mathilde when she received a coded letter that profoundly upset her and her fiancé the day before.
That night, Sinclair set up a stakeout outside her bedroom after Larsan and Rouletabille also fell victim to the sleeping drug at dinner.
They set up a trap with Larsan, who has since woken up, and Jacques, each watching an exit route the attacker can use but he still manages to vanish mysteriously.
Everyone goes back to Mathilde's room when Sinclair spots him again and, aided by the concierge, starts to pursue him only for them to find him dead in the courtyard, stabbed, after he escaped their sight for a brief moment.
Rouletabille informs Sinclair that he will leave for a while to gather evidence as he claims to know the true identity of the attacker, and states the groudskeeper was not the real culprit.
He just comes back from his trip with the evidence but claims he can't reveal the killer's identity before 6.30pm that day.
It turns out that Ballemeyer/Larsan was Mathilde's husband whom she married in secret back when she was living in America and who had to disappear years ago since the police were looking for him.