Kit Owens is a postdoctoral research scientist in the lab of Dr. Lena Severin, who is preparing to begin a groundbreaking study on premenstrual dysphoric disorder (PMDD), an extreme form of PMS. The five postdocs – Zell, Juwon, Maxim, Alex, and Kit – are competing for three spots on the study, when Severin announces a new hire, Diane Fleming. Switching between the present day and Kit's high school years, it is revealed that the summer before sophomore year, she met Diane at a cross-country camp. During an overnight trip, Kit shares a secret with Diane that she had a sexual encounter with an adult, nicknamed Stevie Shoes. In their senior year, Diane transfers to Kit's high school following the death of her father and her mother's move to Florida with her new boyfriend, and the two became lab partners in AP Chemistry. Diane encourages Kit to apply for Severin's STEM scholarship for women.

While studying together, Diane tells Kit she killed her father, with the hope that she would be able to move back in with her mother. Following the confession, Kit refuses to be friends with Diane but, following her mother's advice, does not report it to the police. She instead tells the school guidance counselor, Ms. Castro, who writes the Severin scholarship indicating Diane could have been responsible for her father's death. Kit is awarded the scholarship and attends the state university where Severin works.

In the present day, Kit and Alex go to a bar, where she gets drunk on Long Island iced teas and confesses Diane's secret to him before the two have sex. The next day, Severin announces that one of the PMDD slots is going to Serge, the head lab tech. Diane tells Kit she has been chosen for the other slot but, following her confession to Alex, he messages Kit that he is going to tell Severin about Diane's secret. She goes to the lab the next morning to convince him not to tell Severin and following an argument, he puts too much pressure on a flash column and the test tube explodes, killing him. Unbeknownst to her, Diane is in the lab and, believing that Kit killed Alex, tells her to leave the body to be discovered but removes any evidence that Kit was there.

When they return to the lab, the body is gone and no one knows Alex is dead. The only one who seems concerned is Eleanor, who explains that she is Alex's fiancée. The police are brought in to investigate his disappearance and when they question Kit, they seem to know about her romantic involvement with Alex. She has two realizations: that Zell saw them at the bar and told the police, and that Diane was rejected as a participant in a previous study on PMDD. She goes to confront Diane but discovers Eleanor in the vivarium. Eleanor accuses Kit of killing Alex and after a struggle, a fan is unplugged and Kit hears noises from the ceiling. When Eleanor leaves, the ceiling breaks, dropping Alex's body onto Kit and Diane. Serge admits to hiding the body on behalf of Severin, as they both believed Diane had killed Alex. Diane puts cyanide in Serge's tea, killing him. Severin walks in and proposes that they blame Serge for Alex's death but when the police arrive, Diane confesses to killing Alex, Serge, and her father. Severin reveals that Diane believed she suffered from PMDD and had undergone a hysterectomy. As Diane is leaving with the police, she kills herself with a specimen knife.

Ten years later, Kit is still working for Severin when she receives a letter from Diane's mother, Mrs. Fleming. The letter reveals that Diane had confessed to killing her father but Mrs. Fleming had not believed her. She also sends a photo and Kit realizes Mrs. Fleming's boyfriend at the time was Stevie Shoes and Diane had been kicked out of her mother's house for telling her about his affair with Kit.