A couple of decades after Bride, Dr. Frankenstein's son Wolf (Basil Rathbone) and family come from America to inherit the old Frankenstein estate.
Exploring the reportedly haunted remains of the tower lab, Wolf finds scuzzy old hunchback Ygor (Bela Lugosi) loitering about. Wolf learns Ygor has been tending to the legendary Monster - whose existence Wolf never believed, lying near-comatose in a secret room deep beneath the rubble. Ygor convinces Wolf to restore his brain damaged friend.
Unfortunately, Ygor uses the re-energized giant to take revenge on old enemies.
Climax comes when Wolf has to shoot squirrely Ygor to death, sending the Monster on a rampage. The Monster kidnaps Wolf's annoying young son. To save the kid, Wolf swashbuckles down a rope and kicks the Monster into a heretofore unseen pit of boiling volcanic sulfur.