Flash Gordon (Buster Crabbe), Dale Arden (Carol Hughes) and Dr. Zarkov go to the planet Mongo and fight the evil emperor Ming (Charles Middleton) who is shooting "death rays" at the Earth.
I haven't seen the previous two serials so I have nothing to compare it to--but this was pretty good. It moves quickly and there's lots of action: Flash is thrown into a fiery pit; he's trapped in an avalanche; fights silly-looking robots that explode when hit; is zapped by the "Destroying Ray"; has aerial battles with Ming's warships; is caught in a fierce explosion (twice); falls down a huge mountain AND is almost drowned! WHEW!
But he (logically) escapes being killed each time. Also he's joined by Prince Barin, the Forest Men and even some Rock Men.
I also found this pretty progressive (for its time). Dale isn't just a timid female waiting for Flash to save her--she holds her own in fights and takes over the spaceship a few times when he's wounded. The acting is just OK--but it IS just a serial--they weren't trying to win Academy Awards. Hughes is actually the best out of all of them--Middleton is (easily) the worst. Crabbe looks tired a lot--but he was probably sick of playing Flash again and again. The special effects are primitive but I've always liked those silly little spaceships--they're a guilty favorite! Some of the sets are VERY elaborate (probably used from another movie) and the score is wonderful--until you realize you're hearing the same thing over and over.
The best way to see this is one a day or (like I did) three a day. No way can you watch the whole 4 hours in one sitting!