As the narrative unfolds, Vic Van Allen (Ben Affleck) finishes a solitary bike ride through the lush woods and returns to his home in Louisiana, shedding his cycling gear along the way. His wife, Melinda (Ana De Armas), unexpectedly sees her husband in the nude as he approaches their doorway, highlighting the unconventional dynamics of their marriage. Together, they share a daughter named Trixie (Grace Jenkins) and have seemingly figured out how to coexist, despite the evident absence of passion.
Their social life includes close friends like Grant (Lil Rel Howery) and Jonas (Dash Mihok), with whom they mingle at gatherings, such as the party following Vic's bike ride. During this event, Melinda becomes increasingly attracted to Joel Dash (Brendan C. Miller), a musician who shares her enthusiasm for music, while Vic observes, experiencing a blend of concern and a detached indifference. A fortuitous meeting with Mary (Devyn Tyler) provides Vic an opportunity to express his feelings about their marriage, revealing a tacit agreement allowing Melinda to explore other relationships—as long as she remains in their home and cares for Trixie.
The situation dawns on a tense note when Joel engages Vic in conversation in the kitchen. Here, Vic oddly mentions his "friend" Martin McRae, who has been missing for a month, and cryptically suggests he was responsible for Martin's fate. Joel misinterprets this as an insinuation directed at himself. Consequently, the neighborhood becomes rife with gossip, while Vic insists his comments were mere banter and shouldn't be misconstrued.
As days slip by, Vic and Melinda tread the waters of their lackluster marriage, each well aware of the absence of romantic joy. Their open arrangement offers a sense of liberty as long as boundaries are maintained. Melinda's desire to have Joel over for dinner before he moves prompts an uncomfortable atmosphere during the encounter. After tucking Trixie into bed, Melinda admits her uncertainties about Joel's intentions.
The couple attends a social event where they meet Don and Kelly Wilson (Tracy Letts and Kristen Connolly), whose bond is reinforced by Don's work as an author. When the topic of Martin's disappearance arises, Vic navigates the inquiries with ease. A later moment between Vic and Kelly—a dance fueled by the music—intensifies the atmospheric tension, which ultimately leads to a fleeting intimate moment on their drive home, offering a brief escape from their relationship complications.
As Vic holds a close watch over Trixie during a soccer game, he is unexpectedly pulled away by a concerning call about a strange payment sent to Charlie De Lisle (Jacob Elordi), a captivating piano teacher. Curiosity ignited, Vic follows Charlie to his lounge and becomes enchanted by the young man's charisma. A troubling realization strikes Vic as he suspects Melinda may have begun an affair with Charlie, intensifying his jealousy.
Meanwhile, the local community grapples with the news of Martin's tragic death caused by a gunshot wound, prompting an investigation that intricately intertwines with Vic and Melinda's unfolding drama. During a gathering at Jonas's pool party, Charlie is introduced as Melinda's piano instructor, unknowingly caught in Vic's previous encounter. Tension escalates as Vic observes the growing intimacy between Melinda and Charlie, culminating in chaos when Charlie's lifeless body unexpectedly emerges from the pool, sending shockwaves through the party attendees.
In the aftermath of the distressing event, Melinda's accusation of Vic as the murderer sparks a sense of paranoia as the police investigate. With Melinda convinced of Vic's guilt, she partners with Don to hire a private investigator, David Ricigliani (Joel R Martinez), to spy on Vic, resulting in heightened suspicion as the detective confronts Don about their plan.
Amid rising tensions, Vic's emotional turmoil bubbles over as he encounters Melinda's college ex-boyfriend Tony Cameron (Finn Wittrock), who tries to rekindle his relationship with her. One day, Tony unexpectedly visits the Van Allen home, and Vic, masking his emotions, extends a cordial invitation. But a chance encounter later on prompts Vic to take a darker turn; he lures Tony into the woods under false pretenses, subsequently throwing a rock that fatally injures him, leading to a grim attempt to dispose of Tony's body.
In a chilling twist, Vic tries to keep his family outing—a picnic with Melinda and Trixie—normal, grappling with anxiety when Trixie almost stumbles upon Tony's remains. The eerie drive back home is marked by unwelcome silence, merely interrupted by Melinda's casual remark about forgetting her scarf.
The following day, Vic’s desperate measures lead him to further conceal Tony’s fate. However, Don's unexpected arrival at the scene shatters his plans, and as Vic finds one of Tony's limbs floating ominously in the water, he scrambled to keep his heinous actions concealed. The situation escalates dramatically when Vic, in a reckless pursuit of Don, experiences an accident that nearly costs him his life, while Don's vehicle careens down an incline.
Returning home, Vic faces a cryptic air as Melinda reveals she found Tony's belongings. Yet, in a surprising twist, she chooses to bury her knowledge, remaining silent even as she discreetly destroys remnants of Tony’s existence, an act filled with chilling resonance as both their lives spiral into darkness.
