Debbie Mullen is a brilliant former MIT student with an exceptionally high IQ who gave up her career to become a stay-at-home mother in Hingham, Massachusetts. She now writes the local advice column “Dear Debbie” while secretly cultivating opium poppies in her backyard to produce homemade painkillers. Her husband, Cooper Mullen, is an accountant quietly battling a relapse into alcoholism, though he desperately tries to hide it from his family.

The family’s seemingly ordinary life begins to fall apart when their younger daughter, Izzy, is kicked off her soccer team by the sleazy Coach Robert Pike, who humiliates her by saying she needs to lose twenty pounds. At the same time, their older daughter, Lexi, becomes trapped in an abusive relationship with her boyfriend, Zane, who blackmails her with partially nude photos, threatening to share them unless she has sex with him.

Watching her daughters suffer awakens the trauma Debbie has buried since college, and she quietly begins taking revenge on the people who have wronged her family. After her lazy editor, Garrett Meers, fires her from the Hingham Household following threats of a lawsuit, Debbie retaliates by uploading a video of Garrett having sex with his secretary onto the newspaper’s website. She also settles smaller scores closer to home. After her neighbor Brett Carlson repeatedly complains about her family, Debbie sabotages his electrical system. She later destroys another neighbor (Jo Dolan)’s award-winning garden by placing beetle traps throughout Jo’s prized flower beds. When her pretentious book club acquaintances—Rochelle, Tabitha, and Sloane—mock her and vote to remove her from the group, Debbie serves them sandwiches laced with ingredients that leave them with severe food poisoning.

Before long, her revenge turns far more dangerous. Debbie drugs Coach Pike with brownies infused with opium and plants evidence that frames him for secretly placing a camera in the girls’ locker room, leading to his public arrest. To protect Lexi, she lures Zane to a shipyard, secretly doses his beer with opium, and uses his phone to send police evidence of his earlier child pornography crimes. Intoxicated and panicked, Zane crashes his car into the local high school, breaking his neck and leaving himself permanently paralyzed.

Debbie’s campaign takes a deadly turn after she discovers that Ken Bryant, Cooper’s boss, was responsible for her firing. After Ken’s wife, Cindy, anonymously sought marital advice through Debbie’s newspaper column, Debbie encouraged her to escape her financially abusive marriage and contact a divorce lawyer. When Cindy left him, Ken retaliated by threatening to sue the Hingham Household unless Debbie and her editor were fired. Realizing Ken had destroyed her career simply because she had helped his wife, Debbie confronts him in his home and shoots him in the head.

Everything changes when Debbie’s new gym friend, Harley, confides that she’s having an affair with a married man Debbie initially believes is Cooper. As Debbie investigates, she discovers Harley’s lover is actually Jesse Hutchinson, one of Cooper’s coworkers, who has been pretending to be Cooper to conceal the affair from his own wife, Cindy Bryant. Debbie is horrified to realize Jesse is also “Hutch,” the man who raped her during her sophomore year at MIT.

Debbie also realizes Harley never befriended her out of genuine friendship. Harley had intentionally inserted herself into Debbie’s life to gather information about the man she believed was her lover, showing little remorse for helping destroy another woman’s marriage. Seeing Harley as manipulative and Jesse as the man who ruined her own life decades earlier, Debbie devises a plan to punish them both.

After secretly stealing Jesse’s handgun, Debbie lures him to Harley’s apartment and drugs him with opium until he loses consciousness. She then shoots Harley in the forehead with Jesse’s gun before placing the weapon in his hand and staging the apartment to look like a murder-suicide. Debbie deliberately leaves Jesse alive, knowing the evidence will convince police that he murdered both Harley and Ken Bryant (Jesse’s boss) before attempting to take his own life. As she leaves the apartment, Cooper, who has tracked her using the family’s Findly app (created by Debbie), arrives moments later without realizing what she has done.

One year later, Jesse Hutchinson has been convicted and is serving two consecutive life sentences for the murders Debbie committed. In prison, he is brutally beaten by fellow inmates as payback for another woman he victimized years earlier. Meanwhile, Debbie’s family is thriving. Cooper has started his own successful accounting firm and remained sober, Lexi is attending a prestigious university, and Debbie has become wealthy after developing the Findly app. She also remains close friends with Cindy Bryant, who secretly helped her monitor Jesse’s movements before his arrest. Bound together by everything that happened, the two women share an unspoken understanding that they will never again allow men like Jesse to destroy another woman’s life.