DRAGONBALL Z MOVIE 4 : LORD SLUG

An asteroid alert is wreaking havoc all over the planet as it is going to destroy the entire Earth. Goku and Krillin go to deflect the asteroid off course so it doesn't hit Earth. They have been told they can't destroy it because it is inhabited, and at the time they didn't know who inhabited it. They attempt to move it using a combined Kamehameha, but apparently miss and are blown away by the fierce winds produced by the near-clash of the asteroid with Earth.

The asteroid passes near Earth but misses the crash, exploding soon afterward. However, a spaceship from the asteroid lands on Earth, full of soldiers who serve Lord Slug. Lord Slug is the main antagonist in the movie. Along with his crew of vicious fighters, he traveled from planet to planet wreaking chaos as he went. They attempt to carry out on Earth the same destruction there that they had at a countless number of other planets. The way they dominated a planet was to literally change its atmosphere, freezing the planet. By conforming it to their liking, they made it more suitable for themselves and almost unlivable for any other people, including the native occupants of the planet. Later they transform the planet into a giant spaceship, and move on to find a new planet to conquer.

Upon their landing, a nearby Gohan attempted to stop Slug's soldiers. Slug noticed the Dragon Ball Gohan had on his hat. After reading Bulma's mind, Slug learned about the Dragon Balls and how to get them. Lord Slug had aged hundreds of years and was rapidly approaching the end of his life. So he sent his minions to gather the Dragon Balls, and used them to restore his youth and health; like King Piccolo before him, being fully restored gave him a far greater power.

Goku, along with his son Gohan and Piccolo, squared off against Slug's team and managed to defeat them all. The final battle against Slug was left in the hands of Goku. Goku was able to take Slug on in an incredibly fierce battle. Slug started having the upper hand, until Goku was able to go Super Saiyan (or False Super Saiyan as the form was later named), and started beating Slug.

But Slug then revealed his Namekian nature, and transformed into a giant version of himself. King Kai revealed that Slug was a Namekian who had left the planet a long time ago. But he had a strange mutation that made him pure evil, and was powerful enough to be called a Super Namek. In his new form, Slug started pounding Goku.

The battle was not going well for Goku until he was discarded by Slug who drew his attention away to Piccolo. In a surprising move, Piccolo torn off his ears and told Gohan to whistle. Exploiting a weakness of all Namekians, their extremely sensitive hearing, Gohan's whistling was able to bring Slug to his knees.

Piccolo also gave what little strength remained in him to Goku in his battle against Slug. Thanks to Piccolo, Goku was able to use the Kaio-ken, and managed to punch a hole through Slug's stomach, weakening the Super Namek severely. Goku was able to defeat Slug with ease from that point on. The movie ends with Goku flying up above the clouds to deliver the final blow to Slug with a Genki Dama powered-up from energy taken from the Sun. The Genki Dama heads straight into the freezing generator, taking Slug with it and destroying him.

Source: http://dragonball.wikia.com/wiki/Dragon_Ball_Z:_Lord_Slug