Kay Tracey regards the pale young man in electrified silence when he stares at her and cries out, "Sis! Sis! At last I've found you!" These strange words are the beginning of a baffling search by Kay for clues that will restore the memory of the appealing youth to whom she gives the nickname of Bro. This unexpected guest in the Tracey household, a victim of total amnesia, knows only that he must find his beautiful sister Helene.

Kay's striking resemblance to the missing girl is to lead her through many risky episodes, but the young sleuth is determined, even against threats, to assist the unhappy boy.

Kay has few clues. Why does Bro become frantic upon catching sight of a red scarf Kay plans to wear to a masquerade? And after its disappearance, Kay is plunged even deeper into the dangerous mystery. Her goal is to disprove certain sinister accusations, about which she learns, against Bro and Helene and to bring the brother and sister together.