This was the feature version of the Flash Gordon animated series, from Filmation and NBC. It had actually been in production before the series version. NBC was so impressed with the footage that they commissioned the series, using footage from the feature for stock scenes.
The feature was in production at the same time as the Dino De Laurentis version. In fact, according to Animation by Filmation, (by Michael Swanigan and Darrell McNeil, Blackbear Press, 1993), De Laurentis was also impressed by the footage, and invested money in it when the studio ran into budget problems.
SPOILERS: The film begins in Warsaw, Poland, in 1939, as the Nazis are bombing it. Flash is working for the state department and comes across the name, Mongo. He escapes Poland and flies back to the US on a plane, which is also carrying Dale Arden, reporter. The plane is hit by a meteor and Flash and Dale bail out, when they are rescued by Dr. Hans Zarkov. Zarkov informs them that the meteor shower is a deliberate attack by Mongo, which they must stop. The trio rockets off into space, on their journey to Mongo.
On Mongo, the group meets up with Emperor Ming the Merciless and his daughter Aura. They befriend Prince Thun, of the Lion Men, and make their escape. They form alliances with Prince Barin of Arboria and Vultan, King of the Hawkmen. Together, they launch an assault on Ming's palace and rescue Dale and Zarkov, as well as push Mongo away from Earth.
Over the course of the movie, it is revealed that Ming is the source of the Nazi's greatest weapons, such as the V-2 rocket. This adds a whole new wrinkle to Ming's villainy and makes Mongo an even greater threat to the Earth. Flash is given a government connection and a background in the 1936 Olympics. This is a much better background than an NFL quarterback, as in the other movie.
There are different voices in the film, with a few exceptions. Robert Ridgely is still Flash, Diane Pershing is Dale and Melendy Britt is Aura. Thun is played by Ted Cassidy (Lurch of the Addams Family) and Barin, Ming, and Zarkov have different voice actors. The performances are quite good, something Filmation was not noted for.
I was blown away by this film when it was finally broadcast. It had all of the key episodes and finally provided the beginning of the story, which had been missing from the series. Unlike the De Laurentis film, it kept a serious tone and was faithful to Alex Raymond's wonderful strip. Unfortunately, it was never released on video, although several episodes of the series were. You can still find it from tape traders and, occasionally, on ebay. it's definitely worth seeing if you like the cartoon series or the Sam Jones version.
Review by grendelkhan from the Internet Movie Database.