A nun is found dead by another nun. She claims she was murdered by a resident who died six months ago.

Houdini and Doyle both read about the "ghost killer" in the newspaper and meet at the police station. 

They are reluctantly allowed to assist with the case with Constable Stratton. 

Doyle believes the nun really saw a ghost, while Houdini does not believe in the supernatural.

The nun who saw Lucy's ghost explains that Lucy had a daughter who was taken away from her, providing motive for killing the other nun. 

Houdini discovers missing money and believes the motive was robbery. 

Doyle sees a psychic and asks her to contact his wife. She tells him he's in danger. 

Doyle and Stratton bring down a suspect they believed stole the money. 

Houdini believes it was an inside job.

They learn there's been another murder, sister Mathilda. 

They find a message that says "all things come in threes." 

Doyle finds the missing money and debunks the robbery theory. 

Lucy was Sister Grace's daughter, and the killings were her trying to give her daughter peace. 

Houdini figures out the sound waves from the subway system caused people to think they saw a ghost.