Two thousand years ago, the evil Namekian warlord Piccolo (James Marsters) descended upon the Earth during a solar eclipse and attempted to destroy the world with the aid of his disciple, Ozaru. A group of monks conjured the Mafuba — a powerful but life-threatening enchantment designed to bind those caught within it — and used it to imprison Piccolo. Ozaru disappears, and the Earth gradually recovers. In the present day, Piccolo escapes his confinement and as another solar eclipse approaches, he begins searching for the legendary Dragonballs in order to make a wish to the magical dragon Shen Long for the power to rule the Earth.

On his 18th birthday, a young high-school student and martial artist named Goku (Justin Chatwin) is given the 4-Star Dragonball by his grandfather, Grandpa Gohan (Randall Duk Kim). After returning home from a party hosted by his crush Chi-Chi (Jamie Chung), however, Goku finds his home obliterated and his grandfather near death in the aftermath of Piccolo's failed attempt to acquire the Dragonball. Before he dies, Gohan tells Goku to seek out the martial arts master, Muten Roshi (Chow Yun-Fat), who holds another of the Dragonballs. Along the way, Goku meets Bulma Briefs (Emmy Rossum) of the Capsule Corporation, who was studying the 5-Star Dragonball until it was stolen by Piccolo's servant Mai (Eriko Tamura). Goku offers Bulma his services in exchange for her help in finding Roshi and they ultimately find him in Paozu City. Under Roshi's wing, Goku begins training to harness his Ki, now knowing that they must acquire all the Dragonballs before the upcoming solar eclipse, when Ozaru will return and join with Piccolo. In the midst of the group's search for the 6-star Dragonball, they fall into a trap set by the desert bandit Yamcha (Joon Park) but Roshi convinces Yamcha to join them. Together, the group fight their way through an ambush by Mai and successfully obtain the next Dragonball. As the group continues their quest, they travel to a temple where Roshi consults his former teacher Sifu Norris (Ernie Hudson) and begins training to perform the Mafuba enchantment so he can reseal Piccolo, while Goku must learn the most powerful techniques: the Kamehameha.

During the night, Mai - disguised as Chi-Chi - steals the three Dragonballs that Goku and company have acquired, adding them to the other four that Piccolo has gathered. With the Dragonballs successfully united, Piccolo begins to summon Shen Long, but is stopped by the timely arrival of Goku's team. During the battle that ensues, Piccolo reveals to Goku that he is Ozaru, having been sent to Earth as an infant to destroy it when he came of age. As the eclipse begins, Goku transforms into Ozaru and terrorizes Bulma and Yamcha, while Roshi attempts to use the Mafuba, but is killed before he can re-seal Piccolo. Roshi's dying words restore Goku to his senses, and he engages Piccolo in a final battle, seemingly destroying him with the power of the Kamehameha. Goku then uses the Dragonballs to summon Shen Long, and request that he restore Roshi to life. As they celebrate, they realize the Dragonballs have now scattered, and Bulma declares that they must seek the balls again. Before they head out, Goku visits Chi-Chi so they can truly begin their relationship, but first, they engage in a sparring match to see which of them is stronger.

In a post-credits scene, a woman whom Piccolo spared earlier in the film tends to his wounds as he awakens.