Scotland Yard Inspector Brian Cameron, who has been watching Paula and following Gregory for weeks, figures out that Gregory has been leaving his house at No. 9, then going around into the alley, entering the vacant No 5, then climbing out on the roof and dropping down into the attic of No. 9, where he has been searching through Alice's things looking for her legendary jewels. One night, after Gregory has left the house, Brian pays a visit to Paula. Using the glove that Alice gave him when he was but a star-struck lad, he gains Paula's confidence. When Brian also notices the dimming of the gas light and hears the footsteps in the attic, Paula begins to realize that she hasn't been imagining them. They open Gregory's desk, looking for his revolver, and Paula finds the letter from Sergius Bauer that Gregory told her she had just imagined seeing. Comparing the handwriting on the letter with Gregory's handwriting, Brian concludes that Gregory is Sergius, one of the suspects in Alice's murder. When the gaslight brightens (meaning that Gregory has turned off the gaslights in the attic and is about to return to the house), Brian leaves instructions with Elizabeth and goes outside to wait. Gregory comes in and immediately notices that his desk has been opened. He accuses Paula of doing it, but Paula denies it, saying it was a man who visited the house. Since she doesn't know his name, Gregory calls in Elizabeth to see if she knows him, but Elizabeth denies letting anyone into the house. Suddenly, Brian returns, carrying the dress on which Alice had her jewels sewn. Gregory has been caught in his act, and Brian has Constable Williams (Tom Stevenson) tie him up. Paula asks to speak with her husband one more time alone, during which he begs her to free him. In a turnabout play, Paula pretends that she is "mad", that the knife in his desk is simply imaginary, and that she must have "lost" the knife. Finally, she tells Gregory that she hates him and calls for Brian to have him taken away. In the final scene, Brian and Paula agree to see each other again, and Mrs Tlwaites (who finally gets her chance to see inside the Alquist house) can be heard saying, "Well!"