This movie starts off with Piccolo meditating by a waterfall. Gohan drops by and shows Piccolo Haiya-Dragon's new trick, (he dances to Gohan's whistling). Piccolo gets angry at Gohan when he shows the trick and yells at him. Suddenly Piccolo senses something out in space. Dr. Briefs, who is Bulma's father, sees what appears to be a planet out in space and it's heading straight for Earth! If it hits, Earth will be destroyed! Krillin vows to do something about it as a lazy Master Roshi sleeps.

Goku and Krillin attempt to avert the oncoming planet's course with ki blasts, but they fail and are swept away. Miraculously the planet misses Earth and explodes far off. Everyone is saved, but the thought is short-lived as aliens arrive to conquer Earth. They come in the name of Lord Slug and proclaim him King of the Universe. Lord Slug, a very sickly and old looking alien, sits at his throne with his henchmen right beside him as his soldiers start to take innocent people's lives. Gohan stops the soldiers, and Slug takes notice of something on top of Gohan's hat, a Dragon Ball. He beats up Gohan, Bulma, and Oolong and takes the Dragon Balls, knowing what they can do, and it turns out that Slug is going to use Earth as his Cruiser just like the planet that exploded. He then gets the Dragon Balls gathered and wishes for his youth and power back. After that the aliens block out the Sun light, covering it with thick clouds because all the aliens Except for Slug can't live in direct sunlight.

Goku and Krillin are alive thanks to Yajirobe and his Senzu beans. Later in the night Gohan and Oolong run off to spy on the alien invaders. Gohan is spotted and almost caught, but is saved by Piccolo. They then face Slug's henchmen. Gohan fights Madamatcha, an imp looking alien. Madamatcha is too much for him though. Piccolo kills off his opponent easily and tries to help Gohan but is stopped by another henchman named Angila. Piccolo barely saves Gohan and they are both hurt very badly. Luckily they are saved by Goku and Krillin. Goku easily kills Madamatcha and Angila, but then Lord Slug appears.

Krillin recklessly takes on Slug first and is beaten in one hit. Then Goku and Slug fight, but Goku can't even seem to hurt him. Goku even tries to get energy from the Earth, but it doesn't have any since the Sun has been blocked. All seems lost until Goku gets a sudden surge of energy and starts to beat on Slug. Then Slug decides to show Goku his "true power," revealing that he is a Namek! Slug then uses the namek ability to grow in size. Slug is now gigantic in size. King Kai tells Goku that Slug is not only a Namek. He is also a mutation and a Super Namek. Slug starts to pummel Goku again, but then Piccolo, who is also a Super Namek, joins the battle. Piccolo rips off his own ears and tells Gohan to whistle. It seems that whistling is agonizing to Nameks just like high pitched sounds are to humans. That is why Piccolo yelled at Gohan when he started whistling to make Haiya-Dragon do his trick. The whistling was painful for him.

Gohan starts to whistle and Slug is in agony giving Piccolo the chance to give Goku his power. Goku then uses the infamous Kaio-ken technique to blast right through Slug's chest and finish him and his spaceship off. But Slug reappears and grabs a hold of Goku. Goku, however flies up high over the clouds so he can get energy from the Sun to form a Genki-Dama, killing off Slug once and for all. The clouds finally clear and Earth returns to normal. King Kai thinks Gokou might be the Super Saiyain since he defeated Slug (although Goku doesn't actually become Super Saiyain in the movies ). Finally Master Roshi wakes up from his nap!