The world's largest sapphire, "The Eye of the Daughter of the Moon", was stolen from a royal crypt in China and eventually finds its way into the hands of a gem dealer with a less than stellar reputation. Knowing the gem has a curse placed upon it, the dealer takes possession of it anyway and he receives a death threat, which he shares with his house guest Mr. Wong (Boris Karloff), a detective. That evening the gem dealer throws a party and, during a parlor game, he is shot to death when the lights go out. The home is filled with suspects and Mr. Wong must figure out who is responsible for the killing as well as the true motives behind it, before he or someone else fall victim to the killer.