Flash Gordon: The Greatest Adventure of All (1979)

It is the time of World War II, Flash Gordon is on a mission in Warsaw which is suffering heavy bombing. He arrives too late and his contact is near death. He has a message for Doctor Zarkov, one word "Mongo" before he dies.

Flash travels to find Zarkov and meets 'girl reporter' Dale Arden also on her way to interview him. They are bombarded by meteorites, damaging their plane, forcing them to bail out. They make a hurried escape, chased by lava flows, they find a secret cave and a rocket ship. Doctor Zarkov having no time for introductions, ushers them on board and they blast off. When safely in flight, he explains he is on a mission to the wandering planet Mongo to convince their rulers to call off their attack on Earth .

Before they can make contact, they are shot down by hostile ships and make a crash landing. After barely surviving a large predator's attack, they are captured by animalistic savages and dragged to a giant idol to be sacrifice, from which they barely escape to the outside. There they meet King Thun of the Lion Men.

Flash and company are forced into battle against Mongo's ruler, Ming the Merciless, his daughter Princess Aura, and his robot army of Metal Men. To help their cause, the heroes lead the formation of an alliance beginning with King Thun, leader of the Lion Men; Prince Barin, leader of Arboria; and King Vultan, ruler of the Hawkmen.

Thun explains Ming is too clever to dominate Earth by force alone, and that he would use the Mongo strategy of 'separate and attack' which Flash notes is the same as the Earth expression 'divide and conquer'. Ming reveals he has secretly given military technology to Hitler.

Ming sends his Mole Men to attack the kingdom of Arboria by destroying the roots of the trees of the forested lant. Thanks to Zarkov warning Flash and friends, the attack is repulsed and Flash and his allies use the captured drilling machine to attack Ming's palace. They are overwhelmed by Ming's forces, but Prince Barin insists on his right to a trial by strength. With a flaming sword in hand, Ming duels against Flash in a climactic battle.

As the planet Mongo approaches Earth, they are rocked by quakes threatening to destabilize both planets. To avert the destruction of Earth, they destroy the engine that propels Mongo through space, knowing that without it they will drift though space unable to return to Earth again.