The following version of this book was used to create this study guide: Khong. Rachel. Goodbye, Vitamin. Henry Holt and Company, 2017.

Ruth's diary entries begin the day after Christmas. She writes that she arrived at her parents' home on Christmas Eve to help take care of her father, Howard, who has Alzheimer's disease. She explains that she has recently separated from her fiancé, Joel, after he cheated on her with another woman. She had been under the impression that they were going to move into a new apartment together, but after boxing up all of his belongings, Joel told Ruth that he was leaving her. Ruth is heartbroken, but her best friend, Bonnie, serves as a strong support system for her. On Christmas Morning, Ruth's father shows her a small notebook that he has kept since Ruth was a little girl. It contains letters that he wrote to her about what she said and did on particular days. Throughout the novel, Howard leaves pages of the notebook lying around for Ruth to read.

Ruth tries to convince her brother, Linus, to come to their parents' house with her, but he refuses. She and Linus view their father very differently. When Ruth went off to college, Howard hadn't had a drop of alcohol in years. After she left, when Linus was in the 8th grade, he began to drink again. He ended up having a six-month affair with another professor at the university. Linus views him as a liar, a cheat, and a drunkard.

Howard used to work as a history professor at the university, but was recently fired due to his dementia. One day, Ruth receives a phone call from Howard's teaching assistant, Theo. He and Ruth devise a scheme for Howard to teach a course at the university without him knowing that it is not a legitimate class. Upon hearing the news that he will be able to teach again, Ruth's mother, Annie, insists that Ruth go to the university with her father, so Ruth becomes the assistant for the class. She and Theo spend the semester trying to sneak around the dean without him knowing that Howard is teaching again. Meanwhile, a romance develops between Ruth and Theo.

One day, while rummaging through some drawers, Ruth finds divorce papers signed by her parents two years ago. She approaches Annie about it, but she does not want to talk about it. Ruth wonders if that is why Annie had asked her to stay for the whole year; she wonders if Annie does not want to be alone with Howard. In class soon after, Ruth notices a graduate student named Joan flirtatiously looking at her father. She later discovers that they had had an affair together.

Later on, Howard's class takes place on horseback. Ruth and Theo convince Howard that it is for educational purposes, to learn about the Pony Express. As Howard is lecturing, Ruth notices a couple in front of them on horseback, and anxiously realizes that the man is the dean, Levin. Before long, Levin asks Howard what is going on, and Howard, embarrassed and furious, realizes the scheme that Ruth and Theo have created. He refuses to talk to Ruth or Theo for a week.

Soon afterwards, Joan calls the house, and Ruth picks up. She asks to speak with Howard, because she is distressed that he does not remember her. Ruth, aggravated that she has the nerve to ask to speak to her father, hangs up. Later, Joan leaves a note for Howard, but of course Ruth throws it away before relaying it to him. Then, one day Annie invites Joan and Theo to come over for dinner. Ruth can tell that her father really does not remember Joan. After dinner, Annie, humiliated that Howard cheated on her again and upset with Ruth for not visiting more often, locks herself in her room and does not speak to anyone for days.

Gradually, Howard's relationship with Ruth improves, as does his relationship with Linus and Annie. But of course his dementia gets worse. One day, he spends an hour shouting and accusing Ruth of stealing money from him. After throwing a tantrum, he weeps, apologizes, and says that he wants to help Ruth and his family prepare for when he would become even worse. They proceed to wrap and hide everything breakable and childproof the house, just as they did when Ruth was a baby.

In November, Joel calls Ruth and he tells her that he is marrying Kristin, and that she is pregnant. Ruth, feeling overwhelmed, calls Theo. They go to a bar so that Ruth can relax and they eventually end up playing basketball in a park until the morning. They sit on a park bench together, watching the sun rise over the mountains. The novel ends just after Christmas comes to a close. Ruth, Linus, Annie, and Howard playfully go off into the night, joking around using walkie talkies that Linus bought everyone for Christmas.