Murder by Decree is a 1979 Canadian-British film directed by Bob Clark and starring Christopher Plummer as Sherlock Holmes and James Mason as Dr. John H. Watson. The movie is based on the theory that Jack the Ripper's true identity was covered up by the authorities because he was a member of the royal family.
The story begins with Holmes and Watson being approached by a group of London prostitutes who are terrified of the killer on the loose. They refuse to go to the police because they fear they will be arrested for their profession. Holmes, intrigued by the case, begins his investigation.
With the help of a psychic named Robert Lees, who has received visions of the killer, Holmes uncovers a connection between the murders and the royal family. He discovers that the Prince of Wales had been a patron of one of the murdered prostitutes and may have fathered a child with her. The prime minister, Lord Salisbury, and other high-ranking officials are all Masons and are involved in the cover-up.
As Holmes and Watson try to gather evidence and unmask the killer, they find themselves targeted by the conspirators. The tension builds to a climax as the killer is finally revealed and Holmes confronts the corrupt politicians responsible for the cover-up.
The movie is a suspenseful and atmospheric retelling of the Jack the Ripper story, with strong performances by Plummer and Mason. It presents a compelling theory about the true identity of the killer and raises questions about the extent of government corruption and collusion in Victorian London.