The Murderer Lives at Number 21

The three tenants coordinate their murders so that when one is arrested, another commits a Durand murder while the first is in custody. This creates a perfect alibi system, each time one suspect is imprisoned, a new murder occurs, exonerating them and shifting suspicion to one of the other two. This rotating pattern makes it appear that only one killer exists when actually three people are taking turns.

Earlier scenes show Colin making blank-faced Durand dolls, hinting at his involvement in the calling card motif.

Wens adopts the disguise to get close to the tenants at 21 Avenue Junot, where he believes the killer lives. By posing as a Protestant minister, he can observe the residents daily without raising suspicion. The boarding house becomes his base for surveillance as he works to identify which tenant is behind the Durand murders.

The petty thief's discovery of Durand cards in the boarding house attic directly prompts Wens to move into the building.

Wens deduces that all three tenants are guilty of different Durand murders and plans to arrest them that night. He signals Mila to spread word that an arrest is coming. When the trio learns an arrest is imminent, they seize the opportunity to neutralize Wens before he can apprehend them, abducting him to a nearby building to kill him.

Wens's private conversation with Mila about making an arrest that night provides the catalyst for their desperate action.

Wens discreetly asks Mila to drop hints that he will be making an arrest on that specific night. Though Mila is primarily motivated by wanting publicity for herself, she understands the significance of Wens's plan and brings a contingent of police to the boarding house. Her arrival interrupts the trio's attempt to kill Wens and dump his body in quicklime.

Wens's private request to Mila before the party sets up the rescue mechanism.

Colin, a craftsman in the boarding house, makes and sells unsettling dolls with blank faces. These dolls serve as a twisted signature object connecting Colin to the Durand murders. The calling cards left on victims bear the Durand name, and Colin's dolls, also featuring blank faces, visually reinforce his involvement in creating the killer's identity.

The dolls are introduced early among the tenants' odd talents, establishing Colin as suspicious from the start.

Each time Wens arrests one tenant, Colin, then Linz, then Lalah-Poor, another Durand murder occurs while the suspect sits in jail. This is because all three are complicit; when one is imprisoned, another commits the next murder, creating the illusion of an outside killer and exonerating the arrested tenant. The cycle repeats, frustrating Wens's investigation.

The pattern of arrests and subsequent murders establishes early on that something unusual is happening with the suspects.

Inside the building where they've taken Wens, the trio falls into a heated dispute about who deserves principal credit for orchestrating the complex alibi scheme. Each one explains their individual technique for committing murders while the others are imprisoned. This bickering reveals their narcissistic need for recognition, which ultimately delays their execution of Wens and provides crucial time for Mila and the police to arrive.

The buildup to their confrontation shows three ambitious personalities who cannot share the spotlight.

The trio intends to kill Inspector Wens and dump his body in a vat of quicklime, a chemical that rapidly decomposes organic material. This would destroy all evidence of the murder and ensure Wens's fate remains hidden forever. The plan represents their final attempt to eliminate the only person who has solved their elaborate scheme.

The boarding house setting with its eccentric tenants masks the trio's violent capabilities.