Detective Murdoch and his methods are compromised when one of his colleagues is implicated in a suspicious murder.

With his colleague sentenced to hang for murder, Detective Murdoch races to exonerate them and steps into a trap set by the real killer.

After testifying at Mary Thompson's trial for murdering her husband Percival that the woman is criminally insane and should be committed to an asylum, Julia Ogden receives an anonymous postcard saying she has condemned an innocent woman. Julia and Detective Murdoch re-examine the case and find an anomaly in the time-line but the Crown Attorney isn't interested as he's convinced the woman is guilty. Further investigation reveals that Percival Thompson's business partner, Alexander Wainwright, automatically assumed 100% of their business upon Thompson's death - an agreement they signed just one week before the murder. Wainwright promptly vanishes. With the assistance of the indefatigable Mrs. Brackenreid, who has clearly taken charge, Julia is in the final preparations for her wedding. She and William make it to the altar on the big day but midway through the ceremony they realize they overlooked something very important in the Thompson case.

A murderer claims to be possessed by the spirit of Murdoch's arch-nemesis, James Gillies. Murdoch and Ogden begin to suspect that Gillies may still be alive.