Cath is a freshman at the University of Nebraska–Lincoln, along with her much more outgoing identical twin sister Wren. But Cath is having a hard time adjusting to college. First, because Wren doesn’t want to room with her, and second, because of her social anxiety disorder. A lot of Cath’s free time is spent writing fan fiction for a book series about a boy magician named Simon Snow, and Cath has thousands of passionate followers who read her award-winning fan fiction based on a story similar to Harry Potter. Wren distances herself from Cath as she enters college, professes to have outgrown fan fiction and Simon Snow, and goes out drinking and dancing with her roommate almost every night. Meanwhile, Cath is more interested in her fiction-writing class. The professor is very impressed with her writing, and Cath finds a writing partner and friend in Nick, whom she meets with weekly for writing sessions. However, Cath suffers an emotional and academic setback when the professor gives Cath an F for writing a fan fiction story for a class assignment, as she considers this to be plagiarism.
Cath is, in general, miserable at school. She has no friends, has been emotionally abandoned by her identical twin sister, and gets panic attacks at the idea of eating at her dorm’s dining hall so she subsists on a stash of energy bars she furtively eats in her room. She still suffers from the scars of her mother’s abandonment when she was eight, and she is very worried about her father, who is bipolar and does not take care of himself. So her bold, outgoing roommate, Reagan, and friend Levi decide to befriend Cath. Cath becomes closer to Levi. It turns out Levi is not dating Reagan and is interested romantically in Cath, but when Cath seeks out Levi to let him know she reciprocates his feelings, she finds him in the arms of another woman. Then, Cath is betrayed by her writing partner, who was just using her to get a higher grade in the fiction-writing class. Just before finals, Cath’s father has a bipolar psychological crisis and ends up being involuntarily committed to a mental facility for a short time, causing Cath to ditch her exams and her final paper for her fiction-writing class so she can travel home to Omaha and take care of her father.
Cath becomes very stressed and reaches a point of giving up on school, on Wren, and on the complicated relationships, she has forged at school with Reagan, Nick, Levi, and her fiction-writing professor. However, she decides to continue and starts to learn to deal with her various problems. Meanwhile, Wren’s problems surface in a shocking way. Cath and her sister reunite after Wren gets alcohol poisoning and ends up in the hospital, and while in the hospital waiting to see Wren, Cath sees her mother after years of abandonment. Nick’s betrayal comes home to roost in a satisfying fashion as Wren, Reagan, and Levi defend Cath, and Levi is found to be much more than meets the eye. As the year comes to a close, Cath struggles to finish writing her fan fiction novel of two years, Carry On, Simon, and to write her original short story (as well as a way of making peace with her mom’s leaving her) for fiction-writing.