Pursued by three Indian priests after stealing “The Moonstone” a valuable diamond from the eye of an Indian idol Englishman, Hearncastle is found drowned, presumably by the Indians. Possession of the Moonstone passes to his friend Franklin Blake who presents the diamond to his fiancée. However it is stolen and suspicion falls on another girl who loves Blake particularly after she leaves a mysterious note at her suicide. Subsequently, a doctor discovers that Blake walks in his sleep and took the diamond himself while in that state, but then stolen and never recovered. At last, the thief confesses before his death with Blake and his fiancée reuniting.

• Mystery