In early 2016, Charlie teaches online English writing courses to college students, but keeps his webcam off, ashamed of his morbid obesity. He isolates himself, never leaving his apartment in Moscow, Idaho. His nurse and only friend, Liz, enables him by bringing him unhealthy food, while contradictorily urging him to visit a hospital for a heart failure treatment, though Charlie insists he cannot afford medical care. Charlie is also visited by Thomas, a missionary for the New Life Church who wants to save him. Charlie orders pizza almost every night through an established routine with delivery driver Dan, who leaves the pizza outside on his porch and collects cash payment from the mailbox, the two never interacting face-to-face.
Charlie hopes to reconnect with his estranged teenage daughter Ellie, whom he abandoned eight years ago when leaving his wife, Mary, for a male student of his, Alan. He reveals that he has been saving money for years and offers Ellie the entire $120,000 in his bank account if she spends time with him without her mother's knowledge. Ellie agrees on the condition that he completes all of her homework for her, though she also requests that she write in a notebook he gives her. As Charlie's health worsens, Liz brings him a wheelchair so he can remain mobile.
Thomas again visits Charlie. Liz arrives and angrily orders Thomas to leave, then changes her mind and has a talk with him outside. Liz chastises Thomas for his efforts to help Charlie, then reveals she is the adopted daughter of New Life's head pastor and that Alan was her brother; Alan's religious guilt drove him to suicide, and Charlie turned to emotional eating to cope with his grief, leading to his current weight. Despite Liz's objections, Thomas still believes his mission is to help Charlie. One day, Ellie places crushed Ambien into Charlie's food, knocking him out. Thomas arrives, and the two smoke marijuana, which Ellie photographs, whereupon Thomas confesses to stealing his youth group's money and running away from home because he felt dissatisfied with New Life's mission work there. Ellie secretly records their conversation.
Out of concern, Liz brings Mary to visit Charlie. When Liz learns about the amount Charlie has saved for Ellie, she storms out, furious over having been lied to about why he avoided medical treatment. Mary and Charlie argue over his decision to leave his family for Alan. Mary is also exasperated with Ellie's sardonic, sullen demeanor, but Charlie expresses hope that Ellie will prove he did "one thing right with his life". Later that night, after delivering the pizza and waiting outside in the rain, Dan sees Charlie for the first time, then leaves in a state of shock and disgust. Charlie subsequently has a severe binge-eating episode and sends a profanity-laden email to his students, telling them to disregard the classwork and just write him back > something honest.
Thomas visits Charlie one last time to inform him that he is moving back home after Ellie sent his confession to his former youth group and family, who have forgiven him and implored him to return. He attempts to preach Romans 8:13 to Charlie, but Charlie chastises him when he attributes Alan's death to his sexual orientation and furiously orders him out. During his next class, Charlie tells his students he is being fired for the email he sent, and reads some of their submissions. To reciprocate their honesty, he switches on his webcam for the first time, and the students have mixed reactions. Charlie calmly proclaims that academics and college do not matter, but the honest things his students have written do. He then abruptly ends the class by tossing his laptop against the fridge, destroying it.
Liz returns and comforts Charlie as his health rapidly declines. Ellie arrives to furiously confront him over the failing grade she received on an essay he supposedly rewrote for her, which he secretly replaced with an essay she wrote in eighth grade about Moby-Dick that he considers the most honest essay he has ever read. Ellie initially rebukes him as he attempts to reconcile one final time, but reads the paper aloud at Charlie's insistence. Charlie stands up and begins to walk toward her without assistance, which he had tried but failed to do during her first visit. As she finishes reading, they smile at each other. Charlie begins to levitate, engulfed in a bright white light.
