Leonard Staunton, engaged to Jeanne Baldwin, daughter of a U.S. Senator, is preparing to spend a weekend at the Baldwin estate when Alan Fitzhugh, a club-member, arrives with a note, imprinted with a purple hieroglyph, in which he (Fitzhugh) is threatened. Staunton, telling Jeanne of the plot, stays with Fitzhugh until after midnight, but the next day Fitzhugh's mutilated body is found. Following the funeral, Dr. Mansfield, one of the pallbearers, accidentally smokes a poisoned cigarette; and Staunton, Jeanne, and Detective Condon take him to his home for an antidote; his body disappears, and footprints lead to a slipper with a purple hieroglyph. Numerous other blackmail threats follow, and Jeanne is kidnapped. En route to the ransom rendezvous, Staunton is captured in a speedboat, but a U. S. submarine saves the day and the criminals prove to be none other than Fitzhugh, Mansfield, and Lieutenant Condon.