The world is in a state of panic when it is discovered that a planet has fallen out of its normal orbit and is destined to collide with Earth in a matter of days. Flash Gordon (Buster Crabbe), his girlfriend Dale Arden (Jean Rogers), and renowned scientist Dr. Zarkoff (Frank Shannon) blast off into space in hopes of setting the runaway planet back on course. However, they discover that this planet is inhabitated and ruled by Ming the Merciless (Charles Middleton), despot of the Planet Mongo.

Hurled through boundless space, they land on the onrushing planet and fall into the clutches of the merciless Emperor Ming, who promptly imprisons Zarkov in his laboratory and then, determined to win the beautiful dale for his bride, condemns Flash to fight huge ape-like man killers in the arena.

Finding Flash is an enemy of Ming's, Thun promised to help him, and through a secret passage they entered the palace and learned that Dale was being married to Ming. Flash, trying to reach the wedding chapel, runs headlong into the tunnel of terror and.....

When Aura cuts off the air supply to Kala's underwater palace, Flash rushes to the aid of Dale.

Ming saves the doomed Shark City and delivers Flash and his friends into the hands of the Hawk Men. At the same time, King Vultan plots against the Prince of Justice.

Upon landing in Sky City, Flash and his friends are immediately taken prisoner by Vultan, who demands that Dale renounce Flash forever.

Flash is threatened by Vultan's atom furnaces and electro-charged mechanical devices.

Flash is pitted against his own friend, Prince Barin, in a deadly battle that results in another battle with the Orangopoid.

Outraged by Flash's defeat of the Orangopoid, Ming orders an injection of "drops of forgetfulness" and sends him to the abode of the sacred Fire Dragon.

Flash's memory is gone, but Zharkov accomplishes a memory restoration.

With the aid of an invisibility machine, Flash releases Vultan, and together they prepare to return to Earth, while enemy Aura makes plans to intercept them.

Prince Barin persuades Aura to help with their escape. The Earth party assembles in the Turret House, which is set aflame by fire from a mysterious rocket ship.

Escaping from the Turret House by an underground passage, Flash and his friends return to an electrically protected laboratory. Ming, under attack by Thun's rocket ship, flees to exile in the Palace of Tao.