The Japanese Prince Maiyo is in London to avenge the death of his father who years earlier committed hara-kiri because he had been financially ruined by an English swindler. The Prince warns his friend, the Duke of Devenham, that the Count de la Mar is attempting to seduce the Duke's bored American wife, and then is told by his servant Soto that the Count is the man who killed his father. During a foggy night, the Count, planning to elope with the Duchess, is killed in a taxi with the sword that the Prince's father used to kill himself. Although the American sister of the Duke, Penelope Morse, who loves the Prince, pleads with him to leave before being arrested, he will not perform such a cowardly act. After Soto confesses murdering the Count because he wronged his daughter years ago, the Prince is freed, but because of the racial barrier, he bids a sad farewell to Penelope and leaves.
