Young Edyth Vail, believes her fiancé Allen Morris may have committed a murder, asking Ashton Kirk to investigate the crime. The fiancé admits he went to the David Hume's home to retrieve plans for a submarine, invented by his scientist father and stolen by the murdered man, but Allen claims he left the home before retrieving the plans. Kirk uses a train conductor's punch ticket to discover the identity of the murderers, one of whom is mute, and sets a trap to apprehend the suspects.