A family squabble breaks out at the mysterious Hulstone Towers in Northumberland, England, when brothers Geoffrey and Phillip Musgrave voice disapproval of their sister Sally's affections toward Captain Pat Vickery, an American flyer who is one of a number of military officers convalescing at the estate. Soon thereafter, an attending physician, Dr. Bob Sexton, is seemingly assaulted while walking the grounds. The physician in charge of the facility, Dr. John Watson, then travels to London and asks his good friend, Sherlock Holmes, the famous detective, to investigate the attack. As soon as they arrive at the manor, Holmes and Watson discover Geoffrey's murdered body, a crime which has happened right under the eyes of Inspector Lestrade of Scotland Yard. The police inspector soon arrests Pat for the murder, but Holmes doubts his guilt. The next day, Sally performs the "Musgrave Ritual," a ceremony by which Phillip is anointed the new head of the estate. Later, Holmes questions a drunken Alfred Brunton about the Musgraves, and the old butler is soon thereafter dismissed by Phillip. The next morning, Phillip's dead body is found in the trunk of the family car, and Lestrade immediately suspects Brunton. Realizing the key to the murders may be found in the "Musgrave Ritual," Holmes and Watson search Sally's room, finding the words to the ceremony hidden in a clock. The two then enact a giant chess game on the tiled floor of the manor's main hall, using members of the household as pieces and the ritual as a guide. Holmes then uncovers the ancient Musgrave burial crypt underneath the cellar, with the murdered Brunton inside. Claiming that Musgrave had written his murderer's name in blood by his body, Holmes pretends to go into town in order to procure the chemicals necessary to read the message, but actually waits in the crypt for the murderer to arrive and incriminate himself. Holmes then captures Sexton, arguing that the physician had discovered an old land grant making the Musgrave estate worth millions, killed the two brothers and incriminated Vickery in hopes of marrying Sally, thus inheriting the riches himself. After a brief skirmish, Sexton disarms the detective and confesses all, including murdering the butler, only to discover that Holmes's revolver is filled with blanks. He is then recaptured and arrested by Lestrade and Watson. Afterward, Sally burns the grant, stating that she is not willing to become rich by taking land from innocent farmers and workers. As they drive back to London, Holmes tells Watson that Sally's unselfishness is representative of a new spirit sweeping across England.