Dr. Watson (Nigel Bruce) is staying at Musgrave Manor to assist with several ex-soldiers suffering from different battle psychoses who are rooming there. When the owner of the manor Dr. Sexton (Arthur Margetson) is attacked and wounded by an unknown assailant, Watson summons Sherlock Holmes (Basil Rathbone) to the house to detect who the culprit is, and as soon as they arrive another dead body turns up. As Holmes and Watson uncover more clues Sally Musgrave (Hillary Brooke) is required by family tradition to recite the Musgrave Verse for a ritual performed at a funeral. Believing that the clue to the identity of the murderer is located in the Musgrave Verse, Holmes finds language referring to Chess among the rhyming lines and pulls together the inhabitants of Musgrave Manor to determine what the secret of Musgrave Manor is and what it means to the murderer. When another dead body is discovered in a cavern deep beneath the main floor of the house Holmes makes a bold move to draw the killer out into the open and defeat him.''