Flash Gordon Conquers The Universe (1940)

Starring Buster Crabbe, Carol Hughes, Charles Middleton, Anne Gwynne, Frank Shannon
Directed by Ford Beebe & Ray Taylor

(actor & director credits courtesy IMDB.com)

An attempt by Emperor Ming to spread a deadly plague across the Earth leads Flash Gordon and his friends to return to planet Mongo to thwart Ming's schemes once and for all. 

The third and final Flash Gordon serial returns Crabbe, Middleton, and Shannon as Gordon, Ming, and Dr. Zarkov, but fans will be disappointed the other regulars are recast, notably Carol Hughes replacing Jean Rogers as leading lady Dale Arden.  Still, the fun factor of the serial remains high, with adventures in the woodland kingdom of Arborea, the frozen wasteland of Frigeia, and the desolate cliffs of the Land of the Dead, underscored by classic music cues from Universal's library, particularly the Frankenstein films.  Future Universal horror staple Anne Gwynne has a meaty villainous role as a spy employed by Ming.  There's not as much of an emphasis on fantastic sights as the earlier serials, with the exception of some stock footage used from the first one, but there's spaceship battles, mid-air rescues, and fire and electrical traps that Flash must brave.  Fans of the past entries should find enough to enjoy, although it doesn't measure up to the gold standard of the first serial.  

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.

The Master Key (1945)

Starring Milburn Stone, Jan Wiley, Dennis Moore, Addison Richards, Byron Foulger
Directed by Ray Taylor & Lewis D. Collins
(actor & director credits courtesy IMDB.com)

A government agent and a police lieutenant try to defeat the plans of a ring of Nazi agents in America, who have acquired a machine that extracts gold from sea water.

I wouldn't call this a great movie serial, but it is a fun one, with Universal stalwart Stone and serial veteran Moore trying to track down the secret location of the Nazi agents with the aid of reporter Jan Wiley.  Richards is quite good as the chief Nazi agent, who along with his cohorts all have keys that allow them to communicate with "The Master Key," a mysterious voice that gives them their orders.  The serial could use some more fantastic elements in my opinion, but there's some interesting plot devices in the screenplay, with reporter Wiley employing a group of street kids to do her legwork, a secret relationship between Wiley and one of the Nazi agents, and some twists and turns that always keep the villains a step ahead of our heroes until the conclusion.  Universal horror fans will recognize a number of the music cues, including the title theme which repurposes the main title of House Of Frankenstein.