Lost City Of The Jungle (1946)

Starring Russell Hayden, Jane Adams, Lionel Atwill, Keye Luke, Helen Bennett
Directed by Ray Taylor & Lewis D. Collins
(actor & director credits courtesy IMDB.com)

An agent for a peace foundation is sent to the Himalayan city of Pendrang to track down a warmonger who has financed an archaeological expedition to find a rare element.

This serial from Universal is probably best known for containing the last screen appearance of refined character actor Lionel Atwill, playing the evil and ruthless Sir Eric Hazarias.  Per Wikipedia, Atwill died during the production of the serial, and was ill for some time while he was filming.  As a result, actor John Mylong, who plays Atwill's secretary in the film, had his part rewritten to  make him the power behind Hazarias, replacing Atwill in many dialogue scenes throughout the serial.  This can't help but diminish the serial, as Atwill is the key attraction despite Mylong's best attempts, and with additional scenes of exposition added to further explain his absence, there's less time for action scenes- there isn't a fistfight, long a staple of action serials, until the 5th chapter!  There are a couple of good cliffhangers, although borrowed from past serials, and scenes where the characters go underground in search of the lost city are interesting and atmospheric, although there aren't enough of them.  However, the production's saving grace is when Atwill is on screen- he has some fine exchanges with nefarious dialogue directed at hero Russell Hayden, and the final chapters put him front and center.