Somewhere between dark humor and a psychological thriller. Butter is a morbidly obese (423 lbs) high school senior. Despite years of FitFab summer camps and mom's attempts to get him to eat healthier, he is taking insulin and parking in a handicapped spot because he can't walk from the regular student lot. He uses a special bench to sit on (at a table by himself) during lunch because he can't sit in a regular chair. He always seems to think the worst of others and blames them for ochestrating his problems.
He is a natural saxophone player, but he has no desire to take music classes at school or perform with his teacher's jazz group because he hates visibility. He'd rather stay in the shadows and be ignored. When people do pay attention to him he assumes they're just focusing on negative perceptions (such as how big he is or how much he is eating).
He has a girl in several of his classes he has a crush on, but he knows she'd never be interested in him. So he creates a fake persona from a different school and tracks her down online. They develop a rapport but he resists her frequent requests to share photos. To buy himself some time he promises to meet her on New Years Eve.
However, his life takes a turn for the worse and he comes up with the idea of eating himself to death in a live internet feed. He creates a website called butterslastmeal.com and solicits suggests as to what foods he should stuff himself with. He plans to die on New Year's Eve, at least in part to solve the Anna mystery boyfriend problem. Along the way he uncovers a few cheats to help make sure he is successful (alcohol, drugs, messing up his insulin levels, eating food is allergic to, etc.).
Perversely, as word of his website and plans spreads he becomes popular, invited to hang out with the popular crowd that used to torment him. He goes to parties, goes bowling, does the kind of things he used to only dream of. His new friends are full of helpful suggestions because they've made side bets on what exactly will happen. He eventually realizes that they are befriending him because they know he's dying. He's just going to provide them with some short-term amusement then they'll move on to something new.
At that point he is hopelessly screwed. If he goes through with his plans he will obviously be dead. If he chickens out he will be worse than dead. He will be a bigger social pariah than before, a subject of scorn. The guys who used to harass him are his "friends" right now but if he lets them down by not killing himself they'll make his life hell. Meanwhile, Anna is cutting school hunting for "J.P." at his supposed school, so his cover may be blown before the final day arrives.