The Murderer Lives at Number 21

A petty thief shows Inspector Wenceslas Vorobeychik a stack of Durand calling cards hidden in the attic of the boarding house at 21 Avenue Junot. Wens concludes the killer is one of the tenants and resolves to focus the investigation on the boarding house. The clue tightens the search and concentrates suspicion around the residents.

Wens takes a room in the boarding house disguised as a Protestant minister to observe the tenants up close. He hopes to catch the killer in the act or discern a pattern in the murders.

The tenants present a bizarre cast: Colin makes blank-faced Durand dolls; Doctor Linz is a former abortionist; Kid Robert is a blind ex-boxer; Lalah-Poor is a fakir and stage magician who pockets items. Another resident writes a mystery novel, and some add unusual talents such as whistling. The odd mix makes any one of them plausible as the killer.

As the murders continue, Colin is arrested on suspicion after another Durand murder. Wens notes the pattern and narrows the pool to the boarding house residents, even as the crime continues testing his deductions.

Doctor Linz is arrested as a suspect. Yet another murder occurs while he is jailed, exonerating him temporarily and showing the killer has a plan.

The fakir Lalah-Poor is arrested as a suspect. Then a new Durand murder occurs while he's imprisoned, freeing him and reinforcing the killer's ability to operate within the group.

To celebrate the supposed exonerations, the landlady schedules a party and concert in the boarding house. The tenants prepare performances as the danger persists, and the party becomes a tense backdrop to the unresolved case.

Wens deduces that each of the three tenants is guilty of different Durand murders and plans to arrest them that very night. He discreetly asks Mila to drop hints that an arrest will be made that night.

Just before the party, the trio overpower Wens and take him to a nearby building. They intend to kill him and dump his body in a vat of quicklime to erase the crime.

Inside the building they explain their individual techniques to feign a single killer. They fall into a quarrel about who deserves principal credit as Wens quietly waits.

Mila arrives with a large contingent of police, interrupting the setup and signaling Wens's rescue. The confrontation shifts power as law enforcement closes in.

Wens is saved as police close in and the murderous plan collapses. The would-be trio is confronted with the consequences of their crimes and the illusion of a single killer.

The party resumes in a shaken mood as the police secure the suspects. Wens’s undercover work reveals that all three tenants share culpability for different murders, bringing the Durand case to a close. Mila’s involvement, though for publicity, becomes part of the truth that helps resolve the case.