Harley approaches Cooper at the gym, making him visibly nervous. She insists that she needs to talk to him. He tells her he cannot see her that night, but agrees to come over for dinner the following night. The gym’s receptionist, Cindy, notices the conversation and reminds Harley that Cooper is married. Harley decides to engineer a way for Debbie to discover the affair.

Harley arrives home to find Lisette Inghram, the sister of her last married boyfriend, Edgar, waiting for her. Lisette reveals that Edgar tried to die by suicide after Harley convinced him to leave his wife and then dumped him. Lisette begs Harley to visit Edgar in the nearby hospital, where he is recovering. Harley insists that she is not responsible for Edgar, and then pepper-sprays Lisette.

A letter writer complains that her husband constantly insults her cooking skills, despite the fact that she and her children think the food she cooks is delicious. Debbie suggests that the woman cook with her husband to discover his likes. If that doesn’t work, she suggests poisoning the husband’s food with antifreeze.

Izzy and Lexi announce that Coach Pike was fired for installing cameras in the girls’ locker room. Although no footage was recorded, the app to monitor the cameras was found on his phone. Cooper is shocked to learn that Izzy was kicked off the soccer team and feels disconnected from his family. He grows suspicious of Debbie, and secretly confirms that she was at Coach Pike’s address the night before.

Debbie grows increasingly suspicious of Cooper as he hides phone calls from her. Lexi reveals that Zane has intimate photos of her and is threatening to post them online unless she has sex with him. Debbie promises to handle the situation without going to the principal. The conversation about sexual exploitation evokes painful memories of her own sexual assault in college.

Debbie recalls how she was raped in her sophomore year of college. Although Debbie rarely went out, she agreed to accompany a friend to a fraternity party. Her drink was spiked by an older student named Hutch, who raped her upstairs during the party.

As a result of the trauma, Debbie dropped out of college. She never told Cooper, fearing he wouldn’t respect her. Debbie vows to stop Zane from hurting Lexi the way she was hurt.

Debbie sneaks out at night, anxious about the fact that she was not able to drug Cooper beforehand. She sneaks into Lexi’s room and uses her password to unlock her phone and message Zane to arrange a meeting in a remote park. She considers looking for the photos that Lexi sent Zane, but decides against it.

Debbie spikes a beer with opium and leaves it on a bench in the park for Zane to find. He drinks it immediately and falls asleep soon after. Debbie deletes the photos of Lexi from his phone, then sends evidence of previous attempts to extort girls with revenge porn to the police and school officials. The contents of Zane’s phone make Debbie feel violent, but she is confident that her revenge will be more fulfilling.

Cooper is waiting for Debbie when she arrives home. He demands to know where she has been and indicates that he has been tracking her. Debbie counters by asking where he went two nights earlier. She is heartbroken to realize that they are both lying to each other, and wishes she could tell him the truth. Debbie goes to bed without saying goodnight, and Cooper spends the night downstairs alone.

A letter writer fears that her husband is having an affair, and asks for advice on discovering the truth. Debbie agrees that her husband is likely cheating, and suggests checking for a secret phone and email account. She suggests that the writer hire a lawyer or a hitman if she confirms her husband is cheating.

Debbie wakes Cooper early the next morning so their daughters don’t discover him sleeping alone. They do not speak.

Zane arrives in a violent mood, but is interrupted by Debbie, who describes in threatening detail the violence faced by sex offenders in prison. Zane seems frightened and says he won’t drive Lexi to school anymore. Debbie feels satisfied that she has solved Lexi’s problem.

Cooper decides to confess everything to Debbie in the hopes of closing the gap between them. At work, he learns that a large amount of money is missing from the firm’s account. Cooper’s boss, Ken Bryant, is still absent, and his secretary assumes that he took the money, but cannot confirm it. Cooper worries that Debbie is somehow involved, although he does not know why.

Debbie fantasizes about developing an app designed to help women punish their husbands. She receives a phone call from Izzy, who reveals that a drunk senior crashed his car into the school, sending it into lockdown. After confirming that Lexi is also safe, Debbie rushes to the school to pick up her daughters. She wonders if Zane is responsible for the crash, but dismisses the idea.

At the school, Debbie learns that Zane was the one who crashed his car into the school. Izzy believes he panicked after being called into the principal’s office and tried to flee. Debbie assumes that he got drunk that morning after realizing he was in serious trouble. Lexi is devastated, confusing Debbie, who tells her that her problems are all solved now. Lexi insists that Zane getting hurt is not a solution to her problem. Privately, Debbie rejects responsibility for Zane’s condition.

Lexi refuses to speak to Debbie for the rest of the afternoon. Debbie decides to leave the house for an errand and asks Izzy to keep an eye on her sister. She reflects that Izzy did not cry when Debbie solved her problem by having Coach Pike fired, and wishes Lexi could be more like her. Before leaving, she tries again to speak to Lexi. Lexi tells her not to kill anyone while she is out. Debbie assumes she is joking.

Debbie drives from her home to the campus of the Massachusetts Institute of Technology in Cambridge. The memory of the night she was raped has become so overwhelming that she decides to burn down the fraternity house. She gains access to the house by pretending to be a reporter writing a story about fraternities at MIT.

Cooper spends the day avoiding the suspicious gaze of Ken Bryant’s secretary. In the afternoon, he receives a call from Lexi, who explains that she and Izzy are suspicious that their mother has become a vigilante. They reveal that they found a suspicious document on Debbie’s computer. Cooper insists that everything is fine, but agrees to come home. Privately, he wonders if they are right.

Lexi and Izzy show Cooper dozens of draft replies to Dear Debbie letters in which Debbie suggests the reader kill her husband. They try to call Debbie, but she does not answer.

The girls show Cooper how to search for places Debbie has been in recent days using Findly. One of the locations shocks and frightens him, and he places another call. Frantic, he lies to his daughters and leaves the house.