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.