Doctor Robert Huntington breaks his engagement with Ann Morton, a nurse at the hospital, when he learns that the ill health of his cousin, Jeffrey, may bring him into possession of the Huntington estates. Ann nurses Jeffrey at the hospital and later marries him, though confessing that she loves another. Too late, Robert regrets having given up the girl and tries to win her back. She repulses him. Doubly jealous now of his cousin, Robert schemes to put him out of the way. Ann's discovery of an old volume, "The Crimson Moth of the Huntingtons," suggests a grim expedient. "Whenever the Crimson Moth appears to a Huntington he is doomed," the tradition declares. Robert sees to it that a Crimson Moth appears to Jeffrey just when, having discovered Ann's past relations with his cousin, the last of the Huntingtons is in a frame of mind calculated to receive the full shock of superstition. But the treacherous ivy up which Robert has climbed hurls him down to death, and the curse is lifted.
—Moving Picture World synopsis