Pierre Anthon goes to school on the first day of 7th grade, unpacks his stuff from his backpack, and puts it in his desk.
He realizes that since nothing matters, as he's known for some time, nothing is worth doing. Having realized that, he puts all his stuff back in his bag and walks out of the classroom. Peace out, fools.
He climbs up the plum tree in the front yard of the commune where he lives and decides to get used to just doing nothing for the rest of his life.
When his classmates try to talk him down, Pierre Anthon responds with endless variations on "nothing matters."
After his classmates pelt him with stones and knock him out of the tree, he retreats indoors for a couple of days, but then he climbs back up in the tree, face covered in Band-Aids.
He stays in the tree from August to March, presumably going inside to sleep, change clothes, and occasionally eat something besides plums (we hope).
He continues to torment his classmates whenever they pass his house, taunting them about the meaninglessness of their lives and spitting plum pits at them.
When Sofie goes nuts, Pierre Anthon lets Agnes talk him down out of the tree and take him to the sawmill, where he once again tells his (now bloody from fighting) classmates that their lives are meaningless.
When he turns his back on them, they jump him and kill him.
Pierre Anthon's body burns in a sawmill fire the night of his death.