Long before the rumors of a live-action Planet Hulk movie and the partial Planet Hulk story in Thor: Ragnarok, Planet Hulk was a direct-to-video Marvel animation film. The movie tells the story of a permanently transformed Bruce Banner as the Hulk.
Because of the danger the Hulk possesses, the Avengers decided to send him off to a peaceful planet. As you can guess, something goes wrong. The Hulk wakes up mid-flight and goes into a berserker rage. He destroys crucial navigational instruments and the ship flies off-course.
Crashing landing on the alien planet of Sakaar, he is greeted by aggressors. The Hulk (Rick D. Wasserman) is imprisoned along with other aliens. Their captors force the captives to fight for their freedoms.
Only then do the people realize who the Hulk is. He is the savior of their world.