Flash Gordon, Dale, and a bewildered (yet brilliant) Dr. Zarkov travel to the planet Mongo when it appears that the gypsy planet is set on a collision course with Earth. One of the most enduring of the sci-fi serials.

(Taken from Science Fiction Serials)
Chapter 1: A mysterious planet approaches Earth on an apparent collision course, threatening to destroy the world with its disruptive gravitational influence. As violent storms and meteor showers sweep the globe, Flash Gordon, a young athlete, travel aboard a passenger plane in an effort to reach his father, an astronomer, before the end of the world. Also aboard the plane is beautiful young Dale Arden. When the plane spins out of control in the turbulent winds, Flash bails out with Dale and the other passengers just before the plane is destroyed by a meteor.
Flash and Dale land safely in a remote area, and they are startled to see a futuristic rocketship neaby. Suddenly, a bearded, wild-eyed man appears from the bushes, aiming a gun at Flash and Dale, and announces that he is Dr. Zarkov. Zarkov, a brilliant scientist, claims that he can reach the rogue planet in his rocketship and possibly stop it from colliding with Earth, if Flash will help him. Flash boards the rocketship with Dale and Zarkov.
Zarkov blasts off and, after a swift journey through outer space, the rocketship lands on the mysterious planet. Emerging from the rocket, Flash, Dale and Zarkov are nearly killed by giant reptiles when another rocketship appears in the sky and destroys the monsters with a ray blast. The ship lands and an armored soldier, Officer Torch, accompanied by two armed guards, approaches and announces that they are prisoners of Emperor Ming. Flown to a city in the distance, they are escorted into a massive palace throne room and taken before Ming, the cruel tyrant who rules the planet (called Mongo). Ming, impressed by Zarkov's scientific knowledge, promptly imprisons him in a laboratory and then becomes infatuated with Dale. When Flash objects, he is thrown into the arena on Ming's orders and forced to battle three ape-men as a horrified Dale and Ming's beautiful daughter Aura, interested in the heroic Earthman, watch the fight. Flash overcomes his bestial opponents, and Aura rushes into the arena to protect Flash from Ming's guards when they attack him. Aura shoots one of Ming's soldiers with a ray gun, and the dying man falls against a switch that opens a trap door beneath Flash and Aura.
Chapter 2: Flash and Aura plummet down the shaft, but are saved when Ming throws a switch that opens a safety net beneath them. Escaping from the shaft thru a secret door, Aura leads Flash through a subterranean cave to a rocketship at the cavern opening. Meanwhile, Zarkov is being forced to work on various devices that will aid Ming in his planned conquest of the universe. Ming's hand-maidens for marriange are preparing Dale to wed the emperor; when she resists, Ming's high priest hypnotizes her with a machine called the "dehumanizer."
The lion men, led by Prince Thun, a race in rebellion against Ming's tyranny, attack his palace from the air in whirling gyro ships. Flash, concerned about his friends safety, blasts off in the rocketship and engages the gyros in an aerial dogfight. Flash's rocketship crashes into Thun's gyro as they maneuver across the sky, and they both plunge into a barren valley below. Emerging from the wreckage unharmed, the men begin fighting, then realize that they are allies against Ming and become friends. Stealthily gaining access to Ming's palace, Flash and Thun visit Zarkov in the Laboratory, and are told that the hypnotized Dale is being Married to Ming. Hurrying to stop the ceremony, Flash rushes into a cavern and is attacked by a giant, lobster-clawed reptile that tried to crush him in its grasp.Thun arrives just in time and kills the monster with a ray gun. Flash contin- ues on, disrupting Ming's wedding cer- emony and escaping with the hypno- tized Dale as Ming and his astonished retinue watch. Through a viewing lens, Ming sees Flash and Dale escaping and operates a control that opens a trap door beneath them. Flash and Dale fall into a pool of water and are attacked by shark men who force them to board an under- water ship called a hydrocycle. Dale (now recovered from her hypnosis) and Flash are taken to the underwater shark