The Law of Ueki tells the story of the Battle of the Supernatural Powers, a tournament to decide who will be the next King of the Celestial World (rendered as Kami (???) in Japanese, Godking in the Filipino vesion). Each candidate (100 in total) is required to choose a junior high school student to act as their fighter. Each student is given a unique power--the power to change one thing into something else. The winning King of the Celestial World Candidate (Godcandidate in the Filipino version) will become King of the Celestial World and the winning student will receive the Blank Talent (blank zai), a talent that can be anything they choose. While this leads many students to thoughts of greed and selfishness, Kosuke Ueki has decided to take it upon himself to win this tournament to keep that power away from those that would abuse such a gift.
Initially, fights in the tournament consist of a battle between two power users - one wins by knocking the other combatant out. When a knock-out is obtained, the loser will lose their power and the winner will gain a talent (? zai), which is a “talent” of some sort (e.g. "talent of running," "talent of studying," and so on). A power user who has lost a fight and their power is labeled "retired."
The rules for the combatants of the tournament revolve around the talents:
Power users are not allowed to use their powers against non-power users, or they lose one talent for each hit.
If the power user loses all their talents, then they will disappear.
Everytime a power user knocks out another power user, the one who knocks out the other will gain another talent, senseless or not.
If, for any reason, both contestants become unconscious at the same time, the round is a draw: no talents will be gained, and no powers are lost. The contestants must be knocked out within 5 seconds of each other, or the last contestant wins.
There are also rules which apply to the King of the Celestial World Candidates. The punishment for breaking them is that the King of the Celestial World Candidate is sent to Hell.
King of the Celestial World Candidates are not allowed to interfere with their power user's fight. This means that they are not allowed to provide their power users with more supplies to use their power (this may only apply to during a fight), or even save them from dying.
King of the Celestial World Candidates are only allowed to grant a power to one individual. However, it is possible for two King of the Celestial World Candidates to give a power each to one person; if found out, though, it will lead to a disqualification of both King of the Celestial World Candidates.
Near the end of the tournament, power users are allowed to form groups of five for "battle royale"-style fighting. Knock-outs during this stage do not result in losing powers. Each team is given a penalty if it loses, such as having all of the members die or losing their hair in addition to losing their powers. To decide who receives the Blank Talent among the winning team, a device is used to measure the growth in power that each participant has shown from the beginning to the end of the tournament. To decide who is to become King of the Celestial World, the winning team votes which of the King of the Celestial World Candidates is given that honor.
Level 2 powers may be obtained by the power users. The requirements are a total mastery of their original power and the powerful desire to become stronger.