When a chemical manufacturer is killed after asking detective James Wong to help him, Wong investigates this and two subsequent murders. He uncovers an international spy ring hoping to steal the formula for a poison gas being developed by the first victim's company.
The second in the series of Mr. Wong (James Lee Wong) features starring Boris Karloff finds wealthy gem-collector Brandon Edwards gaining possession of the largest star sapphire in the world, the 'Eye of the Daughter of the Moon', after it has been stolen in China. Edwards, at a party in his home, confides to Mr. Wong that his life is in danger. During a game of Charades, Edwards is mysteriously shot dead and the gem disappears. Unknown to Wong, the jewel is in the possession of Edwards' maid, Drina, who intends to return it to China, but she is murdered also, and the gem is taken again. After one more murder—the suspect list is dwindling—Wong exposes the killer, turns him over to Police Inspector Street, and Wong orders his manservant Willy to return the gem to China.
