Vic (Ben Affleck) and Melinda Van Allen (Ana De Armas) are a couple with a daughter, Trixie (Grace Jenkins), living in the small town of Little Wesley, Louisiana. Vic is a retired robotics engineer, having amassed a great fortune developing the guidance chips used in combat drones. Their loveless marriage is held together only by a precarious arrangement whereby, in order to avoid the messiness of divorce, Melinda is allowed to take any number of lovers as long as she does not desert her family. Even then, their open marriage is made no secret among their friends, while Vic silently grows increasingly jealous of Melinda's lovers.Melinda invites one of her lovers, musician Joel Dash (Brendan C. Miller), to a party. While alone with Joel, Vic tells him that he murdered her previous boyfriend, Martin McRae, who had disappeared a month earlier. The story floats around their social group, causing Joel to move away (he finds a job in Santa Fe, but when Melinda finds out she makes Vic invites Joel over for dinner and apologize to him for scaring him with a false rumor. But late at night, Vic tells Joel that he did kill Martin McRae and send him off in an Uber, before he can get intimate with Melinda) and raising questions as to the story's truth. It is later revealed on the TV news that Martin was found shot to death by someone else.At a social event, Vic has fun dancing with a writer Don Wilson's wife Kelly Wilson (Kristen Connolly). He takes her dancing, and Vic never dances. Vic and Melinda's friends Kristin Peterson (Rachel Blanchard), Jen Fernandez (Jade Fernandez) all notice this. That night Melinda gives Vic a blow job.Vic finds out that Melinda has cut a check for $3000 to a piano teacher Charlie De Lisle. Vic starts following Charlie De Lisle. One morning, Melinda returns home after spending the night with Charlie. Vic wants Melinda to stop seeing Charlie.Melinda's next lover, piano teacher Charlie De Lisle (Jacob Elordi), is invited to a pool party at the home of Vic's friend Jonas Fernandez (Dash Mihok); Vic quietly grows jealous when seeing Melinda and Charlie making love in the swimming pool. Everyone rushes inside when it starts raining, but then find, to their horror, Charlie dead in the pool, apparently from drowning. Upon questioning by police, Melinda is quick to accuse Vic of murdering Charlie, raising growing suspicions by their new friend, a writer named Don Wilson (Tracy Letts). With help from Melinda, Don secretly hires a private investigator David Ricigliani (Joel R. Martinez), who later poses as a psychotherapist. Vic, however, is quick to uncover this plot (when Kelly reaches out to Vic and tells him all about what Don is doing) and crashes a Wilson family lunch, accusing Don of his wrongdoings.Sometime later, Melinda reunites with an old college boyfriend, Tony Cameron (Finn Wittrock), whose flirtatious behavior incites jealousy within Vic. Vic overhears Melinda talking to Tony about moving to Brazil. Vic drives Tony into the woods alone, before pushing him off a rocky hill, killing him. Vic then sinks the body into a nearby creek.This is the spot where Melinda and Vic used to go hiking, back when they were dating. Vic returns home and finds Melinda packing a picnic for the same location, with their daughter Trixie. At the picnic, Vic could see that Tony's body was not submerged enough, and its outlines were visible from the shore.Vic returns to the creek to retrieve Melinda's scarf from a picnic trip they had the previous day, but as he attempts to sink Tony's body further into the water, he is suddenly surprised by Don, who catches him in the act and then recklessly drives his car to flee for help with Vic in pursuit, but in the ensuing chaos (Vic came in front of Don's car and Don actually tried to steer the car to save Vic) Don accidentally swerves his car off a cliff, and is killed instantly upon impact.At the same time, Melinda finds Tony's wallet with his identification in one of Vic's snail tanks, confirming Vic's knowledge of Tony's whereabouts. Melinda begins packing, intending to leave with their daughter, only to be defiantly confronted by Trixie to stay. After contemplating, Melinda burns Tony's identity documents, suggesting she intends to stay with Vic as he returns home.