Gilbert is a 24 year old guy who lives in a small town called Endora. The town is struggling, and Gilbert feels trapped. Gilbert is now the oldest male in his family after his father hanged himself in the basement. Gilbert's mother became addicted to binge eating in her grief, and she is now too overweight to stand up. She is supported by her children.
Arnie is Gilbert's 17 year old brother, and he has a severe mental handicap, such that Gilbert also must care for his almost-adult brother. The doctors said he wouldn't see 10 years old, and they still maintain that he could die at any moment. His birthday is coming up. All Gilbert's mother has ever wanted is to see Arnie turn 18, but Arnie is a handful, and he often threatens his own safety without knowing it. The family also has several sisters, two of whom have moved away already, and two who have stayed. Amy is the emotional leader of the home, and Ellen is a bratty teen who is often the only person passionate enough to address the elephant in the room, but again, she has serious attitude adjustment issues. She also dresses incredibly provocatively and seems sexually mature, but she's significantly younger than Arnie.
Gilbert works as a grocery store clerk and he often makes deliveries for the women in town who don't like to carry their groceries home. There's a lady in town named Mrs. Carver who uses this service as an expedient way to have an affair with Gilbert, and Gilbert loves the sex, but ultimately, he is left disappointed by the arrangement. Plus Mrs. Carver's husband seems to be onto them.
A new girl comes to town named Becky. She's 15, and she's incredibly mature. She and her grandmother were stranded by car troubles and so the set up just outside of town and are friendly to the townspeople. Gilbert has a huge crush on Becky and begins pursuing her. When they meet, Gilbert hears commotion and realizes Arnie has climbed a dangerous tower, like he often does (one of the many ways his mental handicap threatens his safety). Arnie is frustrating to Gilbert because Arnie often acts out when he wants attention. But, also, Gilbert wants to protect Arnie, and Arnie is incredibly difficult to protect, so Gilbert feels paranoid.
Mrs. Carver is very displeased by Gilbert and Becky's new romance. Then her husband has a heart attack and falls face-first in a kiddie pool, and he drowns. Mrs. Carver leaves Iowa forever.
At home, things are very complicated. Gilbert looks like his father, and his mother's weight is so serious that Gilbert thinks she will die soon. She is wearing a hole through the floor, but no one wants to go down to the basement because that's where Daddy died. Arnie climbs the tower again and is arrested, so Gilbert's mother leaves the house to demand his release, and everyone sees how obese she has become. Having recovered Arnie, they prepare for his birthday party. Larry and Janice, older siblings who have moved away, visit for Arnie's birthday. Janice travels the world as a flight attendant. Gilbert dreams of escaping the small town. Gilbert is less jealous of Larry, because Larry discover their father's dead, hanging body, and he left to alleviate his PTSD.
Gilbert has to replace Arnie's cake when Arnie ruins it, but he must go to the new fancy grocery store against his employer's wishes (remember he works at the grocery store as a clerk). That is a huge sacrifice, so when Arnie eats the second cake and ruins it too, Gilbert begins losing his temper and striking Arnie, slapping and hitting his head. Gilbert has betrayed his loyalty to protect Arnie, and he flees in horror and shame. He stays away until the party and when he comes back, his mother is waiting.
Contrary to his expectations, she welcomes him back home and expresses her love for him. She knows that she put unfair pressure on him, and she's sorry. They celebrate Arnie's birthday, and then the next day, their mother has died, but she did the hard work of walking up the stairs, so she was able to die restfully. They know that in order for the funeral home to collect the body will be humiliating, because they will need to use huge machines, and everyone in town will come watch, so instead, the children agree to burn the house with her body in it as a cremation.
