Paris, 1880. A series of unsolved murders, a city panic- stricken, fantastic stories of LE LOUP, a wolf man." Such is the background of the crime involving robbery and murder committed against the banking house of M. DE BRISSON (AUBREY MALLELIEU) and bringing him to the verge of financial ruin. The rich CHEVALIER DEL GARDO (TOD SLAUGHTER) comes to the rescue and in so doing meets and covets CECILE (MARJORIE TAYLOR), the banker's lovely daughter. However, CECILE loves LUCIEN (JOHN WARWICK), a penniless clerk, of whom DEL GARDO promptly disposes by framing him for the bank robbery. When DE BRISSON discovers DEL GARDO'S villainy it is the signal for another murder by the "Wolf Man," and CECILE finds her father stabbed to death. LUCIEN, accused of the murder by DEL GARDO, agrees to meet him in a duel, but DEL GARDO bribes his seconds to knock out his young opponent and throw him into the river to drown. Fortunately LUCIEN is rescued in time to save CECILE, struggling heroically against DEL GARDO'S evil assault on her honor. CECILE seeks police permission for a startling scientific experiment on her dead father by which she hopes to prove that DEL GARDO is his murderer. When the scientist who is to make the experiment is murdered by the "Wolf Man" LUCIEN takes over and nearly tricks DEL GARDO into admitting his guilt. Pursued by LUCIEN and the police, DEL GARDO reaches his home where, caged in an attic, is his half-brother, a creature with the monstrous face and long-drawn howl of a wolf. LE LOUP, the killer Wolf Man who has served his brother well, is a secret which DEL GARDO has sworn shall never come to light. Now, as he drags the aged beast to a trapdoor overlooking the river, a hairy arm clutches him by the throat. The pursuers arrive in time to witness the last of the DEL GARDOS -- the CHEVALIER and his brother LE LOUP -- crash to their death in the swirling waters of the Seine."