Luna and Auguste are college roommates at Walton University, where their friendship gradually turns tumultuous. To mediate the growing feud between them, they seek guidance from the Dean of Student Life, Robyn Schilling. In an attempt to offer them perspective, Robyn shares a compelling story from her past that illustrates how things can always spiral worse than they seem.
Meanwhile, the introverted Devon Weisz is determined to shed her past isolation from middle and high school by starting fresh at Walton. During freshman orientation, she meets the charismatic and confident Celeste, who quickly captivates her with her warmth. By the end of orientation, Devon invites Celeste to share a room in the honor student dorm, and Celeste enthusiastically accepts.
At first, their relationship flourishes; they bond with minimal conflicts while making plans for an exciting spring break trip to Panama City with their dorm friends. Devon decides to cover the trip costs using her parents' emergency credit card and finds herself crushing on Michael, who surprisingly turns out to be her teaching assistant at a lively house party.
However, the chemistry between Devon and Celeste takes a turn during the intense midterm season. When Celeste claims to be unwell, Devon selflessly writes her paper, only to discover that Celeste is perfectly healthy, engaging in intimate activities with a boy in their shared room and accidentally demolishing Devon's project in the process. Distressed, Devon voices her grievances to her parents, who suggest inviting Celeste home for Thanksgiving, highlighting potential issues in her life.
During the holiday, Celeste quickly endears herself to Devon's family, including her younger brother Alex. While trying to prove responsible by keeping an eye on the deep-fried turkey, Devon gets momentarily sidetracked at the insistence of her grandmother, Gigi. Unfortunately, this leads to a culinary disaster when the turkey explodes, which prompts Celeste to insinuate that Devon lacks responsibility.
Returning to campus, the tension in their relationship escalates as Celeste continues her passive-aggressive behavior by revealing Devon's antidepressant prescription and pilfering her wardrobe. Seeking to resolve the situation, Devon meets with their Resident Assistant, who is a younger version of Robyn, to request a room change. Unbeknownst to her, Celeste discovers Robyn's email regarding this switch and later publicly humiliates Devon by reading a scathing poem in their poetry class, aimed directly at Devon's introversion and anxiety.
As Devon grows closer to Michael, she receives encouragement from her professor to participate in WaltonCon, a campus-wide event aimed at improving student life. A kiss shared between Devon and Michael at a party is abruptly interrupted when Alex becomes ill after ingesting what turns out to be MDMA supplied by Celeste, who falsely claimed it was harmless Dramamine.
Spring break finally arrives, and in an unexpected twist, Celeste signs Devon up for karaoke. Though initially anxious, Devon finds her confidence and impresses the crowd, while Celeste spitefully kisses Michael. Heartbroken, Devon returns to campus where Alex comforts her, and during an unexpected encounter involving a fake ID handed to her by Celeste, she uncovers the truth about Celeste's background. Devon learns that Celeste's father, John, is actually the CEO of Staples Inc., and she grew up in a lavish mansion, contradicting the poor image she portrayed.
In a quest for revenge, Devon and Alex plot to use the WaltonCon presentation as a platform to expose Celeste's deception under the guise of enhancing dorm life. However, the rivalry escalates dramatically when Celeste moves out, retaliating by humiliating Alex at his high school through a fabricated prom proposal to his crush, Peter.
Tempers flare when Devon confronts Celeste in Robyn's room. This confrontation leads to a physical altercation, where Celeste reveals that she had been secretly involved with Michael for months, which further infuriates Devon. In a fit of rage, Devon resorts to burning Celeste's belongings with a blowtorch that Robyn provided, inadvertently igniting a fire that consumes the entire dormitory. In the chaos, Celeste confesses her fabricated life story, sharing her struggles, including a mother suffering from Alzheimer's and a father who abandoned her for a younger woman. Devon reassures her that she would have offered support had Celeste only been truthful.
Following the dramatic turn of events, emergency responders arrive on scene, including police and firefighters. In an ambulance, John confronts Celeste, asking her what her next steps for college will be, only for Celeste to adamantly insist that she wants to go to rehab for PTSD, a misunderstanding that John interprets as a diagnosis of a venereal disease.
As the story reaches its conclusion, Luna and Auguste reconcile, and Robyn reflects on the fates of the students. She reveals that Celeste faced expulsion on two additional occasions, was financially severed from her father, and now works at a Staples store. Michael's culinary ambitions culminated unsuccessfully in a restaurant named Miguel's. Meanwhile, Devon was sentenced to two months in prison for her actions, where she formed a close friendship with her cellmate, Louise. After their release, they launched an architectural and design firm based in New York City. In contrast, Alex chose to forgo college and is now engaged to Peter.
