Spaceship to the Unknown (1966)

A heavy condensation of the original serial compresses the original thirteen episodes into an efficient 97 minute feature. Disaster seems imminent when scientists discover that the planet Mongo is about to crash into Earth. Luckily, heroic young Flash Gordon is on hand to lead an investigative mission into outer space and onto the speedily approaching planet. There, he and his best girl, Dale, who is along for the ride, learn that Ming, the devious ruler of Mongo, has purposely put the planet on a collision course with Earth, and only Flash can stop him.

Unfortunately the movie Spaceship to the Unknown is not yet available on HBO Max.