Flash Gordon, Dale Arden, and Dr. Hans Zarkov, visitors from Earth, are traveling in a spaceship to the planet Mongo with a dire warning: The two planets will soon collide. However, as the ship approaches, the group is shot down by local forces, crashing in the ocean. They are saved by a group of Gill Men, insofar as they are saved from drowning; they effectively become part of the "catch of the day," alongside Prince Barin of the Arborians, and Prince Thun of the Lion-Men. The two of them, little as they can appear to stand one another, give them their introduction to Mongo: The planet is suffering under the tyrannical rule of self-proclaimed Ming the Merciless. However, all races on Mongo are enemies to one another; as a result, Flash observes, no matter how much any or each of them may hate Ming, they can never stand together against him.
The group is brought to a cave, where it seems as if they really will be treated as the catch of the day until Prince Barin hijacks a vehicle and tries to escape. He is successful, only delayed when Flash manages to board the craft, slowing him down long enough for everybody else to get aboard. Barin takes them out as far as the swamp, then detaches the back of the craft and takes off, leaving the rest of them stranded aboard their makeshift raft, stuck with minimal propulsion, and easy prey for a new threat: Princess Aura, Ming's daughter, and a huntress (backed by her "Witch-Women") second only to Prince Barin himself.
They flee to Dire Swamp, where Thun warns the Earthlings that nobody can live there. Still, with the only alternative being captured and risking the mercy of Ming - and he has none - Flash decides to take the chance. Trudging through the swamp, they are menaced by a variety of creatures, but remain unharmed until they reach dry land... and a carnivorous plant up in the trees that threatens to consume them, grabbing both Flash and Thun in its tentacles. They are only saved when Aura catches up, stunning the plant with a special rod.
With her prey safe, Aura takes them all prisoner, up until the moment when Flash overpowers her, depriving her of her weapon, ordering her fellow hunters to stand down. The gambit nearly saves them, but for the timely arrival of Captain Eric and a squadron of Metal Men on the scene. The tables turned, Flash and his friends are forced to board the ship. The trip lasts just long enough for Aura to make her designs on Flash clear (as well as catch a glimpse of Sky City) before they reach Mingo City; at its heart, Ming's palace; and in the center of that, Ming's throne room.
Dr. Zarkov speaks, hoping to warn Ming of the coming collision. Ming is dismissive. He assures them that their planets will not indeed collide. When Zarkov protests the disastrous consequences of even a near miss, Ming agrees, and reveals that that is in line with his plan: Mongo (moving under his propulsion and protected by his own devices) will wreak havoc on Earth, leaving it in chaos and ripe for conquest. While the Earthlings are horrified by this, Ming quickly lays out his will: Zarkov will join his scientists; Dale, his other women. His daughter Aura desires Flash for herself, but Ming first chooses to test his abilities in the arena. Pressing a button on his throne, he activates a depression in the floor, pitting Flash and Thun against what the lion-man recognizes as a "training ball" - a large sphere, rolling about and shooting a form of laser fire at them.
The combatants are chased around their circular arena by their robotic opponent, forced on the defensive, until Flash somehow gets his feet under the thing and, using its momentum against it, flings it out and into one of the throne room's pillars, cracking it. In the following confusion, Dale is able to break free of her captors, and Princess Aura is able to press another button on Ming's throne, opening up a pit in the floor - their only chance at freedom. Flash, Thun and Dale are just barely able to jump through before the doors close again, falling through a deep pit into a cave. Not far from where they have landed, they see a way out - but closer still is the shadow of a massive reptilian beast...