When Elvira Cummings, a young opera singer, is poisoned during a rehearsal of La boh?me, Detective Murdoch suspects the intended victim was Madame Rosa Hamilton, the diva who stormed off stage moments before her prot?g? collapsed. Showing her thoroughness and skill, Dr. Grace's autopsy brings her to the same conclusion as Murdoch: the aspiring soprano was killed by a fast-acting poison, likely slipped into her prop glass by one of the other on-stage performers. A serious opera aficionado, Inspector Thomas Brackenreid takes a personal interest in the crime, and the famed and flirtatious soprano, inflaming the jealousy of his wife Margaret. Further investigation into the opera company uncovers almost unanimous disdain for Rosa, and a rumoured love triangle between Rosa, her longtime lover, tenor Carlo Corsi, and the dead singer. As Murdoch, Brackenreid and Constable George Crabtree delve for answers, the dramatic behind the scenes machinations reach operatic heights.
The series takes place in Toronto in the 1890s and follows Detective William Murdoch of the Toronto Constabulary, who solves many of his cases using methods of detection that were unusual at the time. These methods include fingerprinting, blood testing, surveillance, and trace evidence.
Detective Murdoch is assisted by the three other main characters: Inspector Brackenreid, Doctor Julia Ogden, and the inexperienced but eager Constable George Crabtree, who aspires to be a mystery-novel writer. Brackenreid, Murdoch's immediate superior, is a blunt and sceptical Yorkshireman with a fondness for whiskey, and prefers conventional methods of detection over Murdoch's eccentric methods, though he is typically pleased and proud when Murdoch is successful despite the odds. Crabtree is often unable to grasp the more advanced methods, but his enthusiasm and loyalty make him a good assistant. Like Crabtree, Dr. Ogden is a great supporter of Murdoch's methods. Her skill in pathology usually helps by revealing a great deal of useful evidence to aid Murdoch in solving cases.
Throughout the series Murdoch's growing infatuation with her, and his inability to express his feelings, provide a light subplot. In the fifth season, after Dr. Ogden is married to Dr. Darcy Garland (a colleague she met in Buffalo), a new doctor is introduced, Dr. Emily Grace. She and George Crabtree show some romantic interest in each other.