On the advice of his friend Pendleton, Ashton Kirk, a scholar and amateur detective, pays a visit to a house whose occupants, Charles Cramp and his sister Grace, complain of strange happenings involving Mexicans. Discussions with Ashton's agent in Mexico reveal that Cramp's father was once an engraver, who, in desperate need of cash, had agreed to supply Alva, a notorious thief, with forged currency plates. After a change of heart, Cramp refused to deliver the plates to Alva, and now Cramp's aunt, Miss Hohenlo, has come to her brother's home to find them herself. The clever Ashton realizes that the Mexicans are cohorts of Alva's and eventually uncovers an elaborate signalling and tunnel system used by Miss Hohenlo and Alva to locate the missing plates. Deciphering a message announcing the time and place of the arrival of the Mexicans, Ashton and his aides hide themselves in the house cellar, capture the thieves and destroy the plates.

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.

Ashton Kirk is a wealthy and scholarly young man who solves mysteries that have the police puzzled. Stella Morse asks Kirk to find out who is threatening her uncle, Dr. Morse. Morse has a copy of a secret treaty between Russia and the United States that was made during the Russo-Japanese War. When the Doctor is murdered Kirk takes on the case. Japanese spies have been trying to purloin the treaty to ruin relations between its country and America. Kirk manages to get the treaty himself and prevents a war from breaking out.