Holmes' brother Mycroft joins him to investigate the murder of Arthur Cadogan West, a clerk in a government defence office found dead by railway lines with pages from a secret submarine plan in his pocket. Holmes believes that the victim was killed somewhere other than the railway track. Discovering a suspect whose house backed on to the line and could have dropped the body onto a passing train roof leads him towards the culprits who really stole the plans and sought to incriminate an innocent bystander.