In a flashforward to January 2016, Zayday (the president of Kappa) and Grace (her vice-president) give a speech to the assembled pledges. Dean Munsch also appears, telling everyone that she is glad that the ugliness of the fall semester is behind them. Zayday and Grace introduce an eye-patch wearing Hester as their treasurer. In a voiceover, Hester explains that she stabbed herself in the eye as part of her twenty-year revenge plan in the making and that she got away with the murders. She also mentions that she is the only Chanel left in Kappa.
In a series of flashbacks, Hester explains how she and Boone grew up in the asylum with Gigi, who raised them and trained them to be effective killers. She also explains that Boone never officially enrolled in college, but that she disguised herself as a girl with a neckbrace in order to enroll and fly under the radar (since no one asks strange people any details about their past). Gigi and Hester spotted the Wallace University mascot and murdered him first to take the costume for their disguise.
Later on, Hester and Boone went to Kappa house and switched out Melanie's spray-tan with acid. Hester explains that Boone was an idiot and Gigi was a psycho with a black heart, but that she (Hester) was the brains of the operation. Hester was admitted to the university based solely on the fact that she was wearing a neckbrace (despite her file clearly being fake and Dean Munsch knowing that).
Hester gleefully explains that she now has a family, despite the fact that Grace and Wes don't know that they are her half-sister and father. She is glad to be rid of Boone and Gigi, who she considered crazy.
In a flashback to December 2015 (the present), Hester explains by voiceover that she stabbed herself in the eye, avoiding any permanent damage by studying Gray's Anatomy. As she is taken away on a stretcher, she again points at #5 and declares that she is the killer.
Back at Kappa house the Chanels accuse #5 of being a killer, and discuss the evidence that she committed the murders. Denise shows up at the house, as does Hester wearing an eyepatch. Grace tries to insist that Hester is the murderer, based on her fake admission file. Hester explains away the fake file and brings in fake parents that she claims were in the CIA, in order to explain there being no records of her parents. Hester claims that #5 is the bathtub baby and Boone's twin sister.
Chanel #5's parents also arrive at Hester's behest and instead of backing up their daughter's insistence that she was not adopted and is not the killer, they say that she was given to them by a woman named Gigi who told them to enroll #5 in Wallace University when the time came, and that they fully believe she is the murderer. In a flashback to two weeks earlier, it is revealed that Hester met with Chanel #5's parents, who readily agreed to lie about #5 because they dislike their daughter for being so lame.
Though she still voices her belief that Zayday is the actual killer, Denise announces that due to the overwhelming evidence, she must arrest #5. Hester interrupts by announcing that she must also arrest #3 for being the killer.
Hester claims that #3 killed both Deaf Taylor Swift and Predatory Lez. She produces letters from #3's biological father Charles Manson, in which he encouraged her to kill her sorority sisters. #3 tries to explain that she tried to ask her father for normal young woman advice but instead he only responded with messages about murder. Hester reads aloud a letter suggesting that #3 kill a pizza delivery guy by strapping a bomb to him, mirroring what actually happened earlier that week and casting further suspicion on #3. She also claims that #3 has a split personality named Dirty Helen and produces "proof" in the form of a letter from a psychiatrist saying that Dirty Helen threatened his family and drove him out of the country. Hester claims that Dirty Helen met with #5 and they plotted the murders together. #3 admits that it could have happened because she wouldn't remember it, but #5 insists that the conversation never happened.
As Denise prepares to arrest both #3 and #5, Hester also announces that Chanel Oberlin needs to be arrested for murder. She presents video footage of herself in a blonde wig buying gardening shears and a crossbow, among other murder items, with Chanel's Diners Club card, claiming it is proof of Chanel being the killer. Chanel denies having bought the items. Zayday brings up that they did all see Chanel murder Mrs. Bean (and a flashback reveals that Hester turned on the fryer). #3 also brings up the fact that she just saw Chanel attempt to murder Melanie Dorkus.
Though Chanel denies having any motive, Hester claims that Chanel saw an opportunity once the murdering started to off any of the pledges that were different from her who she perceived as ruining her sorority. Denise applauds Hester's ice-cold logic and Chanel is cowed. Denise calls in back-up (strippers dressed as cops who Denise deputized) when the Chanels attempt to escape. The Chanels are all arrested. Denise follows them out, announcing that she still has her eye on Zayday Williams.
Dean Munsch, in another flashforward, reveals by voiceover that the Chanels were sent to jail awaiting trial when their bail was revoked due to their being in contempt of court. She is gleeful about it. She also recounts that Chad and Denise got together for real, but broke up when she went off to Quantico to join the FBI. Chad was heartbroken and they said an emotional goodbye.
At the Dickie Dollar Scholars house, Chad tells the assembled reporters that he has established a foundation and is donating all of the proceeds to charity, in honor of his dead frat brothers (though he can't explain what charity the proceeds are going to).
Dean Munsch had a ghostwriter write a book on New New Feminism for her and is now a national celebrity and NYT best-selling author. She sums up New New Feminism by saying "women are better," to the applause and enthusiasm of her assembled female readers.
Dean Munsch sees Zayday, Grace and Hester cleaning off the memorial for the killed students, which was donated by the Radwells and has a silly inscription written by Chad. Grace and Zayday go off, with Grace explaining that they are having a baking party. Dean Munsch confronts Hester, admitting that she knows Hester was the real killer. She tells Hester that all of those kids didn't deserve to die. Hester claims that if she had grown up with a great dad like Wes, perhaps she would have been more sane, and that she didn't personally kill anyone except for Pete (saying that that was allowed because he too was a killer).
Hester insists that the Chanels deserve to be punished because they were part of the problem (the Greek system) that killed Hester's mother and made Hester what she is. Hester tells the Dean that she is never going to murder anyone again. Dean Munsch tells Hester she is planning to turn her in. Hester threatens to tell everyone about the crimes Munsch committed, including covering up Hester's mother's death and killing her own ex-husband, but suggests they both just forget what they know and part ways. Dean Munsch agrees to stay silent in exchange for Hester's silence.
Grace explains to her father Wes that the Kappas established a hotline for pregnant girls, in order to avoid anything that happened to Sophia Doyle ever happening again (though the hotline is not in much use). Wes tells Grace that he is finally giving Grace the space and trust she needs to be a regular college student by going to Napa with Cathy. They hug goodbye.
At the Chanels' trial, the jury initially finds the Chanels not guilty but switches the verdict to guilty after Chanel rails against the jury and insults them all. The Judge lectures the Chanels, calling them the worst people ever and insane for having chosen to represent themselves in a capital murder case. He sentences all of the Chanels to the Palmer Asylum for the Insane. Chanel has an epiphany that perhaps they are all crazy sociopaths and just never knew it because they surrounded themselves with other crazy people in Kappa.
Chanel explains by voiceover that the Chanels fit right in at the asylum. Chanel #3 engaged in a lesbian affair with one of the nurses, and #5 went on medication that made her a more tolerable person and became Chanel's best friend. Chanel admits to #5 that she loves it at the asylum and never wants to leave. #5 agrees with her. Chanel states that the best part of the asylum is that there are no boys to stay skinny for, so they can eat whatever they want.
The Chanels organize a vote among the asylum women, voting Chanel in as "house president."
As Chanel happily goes to sleep that night, she wakes up suddenly to see the Red Devil standing above her brandishing a knife and screams.