El Capitán Marte y el XL5

A spacecraft carrying an atomic device much more powerful than a hydrogen bomb approaches Earth. The World Space Patrol sends Fireball XL-5, piloted by Steve Zodiac, to intercept. It turns out the spacecraft is manned and is carrying out a suicide mission. Zodiac destroys the spacecraft and is instructed to travel to the craft's source -- Planet 46.

While dodging a runaway planet out of its orbit, the crew of Fireball XL5 discovers it is on a crash course to the planet Membrono. Assuming the planet is deserted before the crash, Steve discovers after seeing a flying saucer that this planet needs help to avoid the fatal impact.

A duo of wacky aliens, the Lilispatians, captures the commander of XL-7 for hostage to trap the upcoming "Mayflower 3" immigrant spaceship, in order to turn the occupants into slaves for their planet.

The evil Dr.Rootes provoke an incident at Space City Power Plant in order that an alien space ship bring a plant seed which invades the planet. Steve, Venus and Matt must accompany Dr. Rootes to Planet Hedra in order to get an antidote to this sudden vegetal invasion.