Phantom of Chinatown (1940)

This is the last of the Inspector Wong movies though Wong was not played by Boris Karloff this time but Keye Luke. An East Asian lead character being played by an East Asian actor was rather novel for the time. This also seems to be a prequel as Wong and Captain Street (Grant Withers) appear to meet for the first time.

Dr Benton (Charles Miller) has returned from an expedition to Mongolia where he has apparently found a secret tomb by an ancient Chinese emperor and a scroll that reveals a mysterious secret. Benton is killed by poison during a presentation. Wong and Street begin the investigation assisted by Chinese agent Win Len (Lotus Long). They discover that the secret, now missing, scroll reveals the location of a massive oil deposit...

And it is all rather enjoyable. A neat little mystery with plenty of suspense and misdirection. There is the added fun of the actor who was usually Charlie Chan's bumbling "number one son" leading a detective case!