Basically a boy in a competition he doesn't know about, sponsored by a celestial being in a bid to become god, gains the power to turn trash into trees. He actually picks this power in order to help fix the environment and ignores everything his sponsor tells him about having to fight others and stuff. He then essentially becomes fate's whipping boy to the great amusement of the audience.
Then a long list of big bads come after him one at a time in order to knock him out of the competition, each with more random, obscure and just plain useless powers. All of which he somehow manages to beat by attacking them with trees.
The plot is terrible, the voice acting absolutely blows and half the time it's unclear why anyone is actually doing what they are doing, or what motivates them to take part in the competition at all.
That said this show is so addictive it's like a sick joke. The fights are astounding, the situational jokes are hilarious and, not to put this lightly, the main character regularly beats people up with f'ing TREES. How? You may ask, but I shall not reveal. Watch it yourself. Seriously, you won't regret it.
I'm going to go watch it again.