In the opening scene, we meet Beth Slocum, portrayed by Aubrey Plaza, as she strolls through a quiet forest, absorbed in her phone. Meanwhile, her boyfriend, Zach Orfman, played by Dane DeHaan, finds himself in a supermarket desperately searching for black napkins, only to settle for beige. Tragedy strikes when Beth dies from a snake bite, and Zach attends her funeral, receiving condolences from friends and family.
Later, Zach visits Beth’s home, where he engages in a game of chess with her parents, Maury and Geenie, brought to life by John C. Reilly and Molly Shannon, respectively. Amidst the grief, Maury lights up a joint, sharing it with Zach, who confesses that their relationship had its struggles prior to Beth’s death. Maury imparts wisdom, stating Beth’s last words to him were to stop being annoying, urging Zach not to let fleeting moments define their entire relationship. As a parting gift, Geenie hands Zach one of Beth's scarves, urging him to cherish her memory.
Despite his family’s advice to move forward, Zach remains in emotional turmoil. The Slocum family begins to cut ties, ignoring his attempts to reach out. One day, driven by curiosity and desperation, Zach peeks through the window and is convinced he sees Beth wandering inside. Alarmed, he pounds on the door, only to be confronted by his brother, Kyle, played by Matthew Grey Gubler, who threatens to have him arrested for his antics. Compounding his anxieties, Zach visits Beth's grave to find an unsettling hole in the ground.
Eventually, Zach manages to gain entry into the Slocum house, where Maury tries to shoo him away. However, the shock sets in when Zach discovers Beth is actually alive, claiming amnesia regarding her death because she is preoccupied with upcoming tests. Misunderstanding Zach’s inquiries, she mistakenly believes he suspects infidelity or a breakup. Ignoring her parents' warnings, Zach indulges in moments with Beth, which include swimming and secret rendezvous at the park. However, he notices the snake bite on her skin and the aftermath of her newly formed blister as she returns home.
One fateful evening, the couple heads to the beach where Zach serenades Beth with a song he penned himself, only for her to react negatively, which instigates a chaotic outburst. In the heat of the moment, she vandalizes the lifeguard stand, igniting a fire. Attempting to diffuse the situation, Zach quickly retreats to the car, but in a fit of rage, Beth shatters the window with her elbow, fearing abandonment.
As strange behavior sweeps over the town, with residents echoing Beth's erratic mood swings, Zach finds solace at a diner where he meets an old friend, Erica Wexler, portrayed by Anna Kendrick. Their conversation grows uncomfortable as Zach struggles to articulate the bizarre circumstances surrounding Beth’s revival, leading to unexpected confessions about his wish she was still dead.
In a moment of sheer panic, an accident occurs as Zach accidentally runs Beth over with his car. Although she appears unharmed, her reaction is nothing short of terrifying as she scares away bystanders. Realizing the need for confrontation, Zach brings Beth to her grave to explain the truth about her demise. Her furious response leads her to hijack his car, leaving Zach to walk home only to find mayhem: his grandfather has reanimated in their living room alongside the cranky former residents of their house.
With Kyle frantically shooting at the bizarre undead, Maury accuses Zach of causing this chaos by revealing the truth to Beth. They deduce the need to consult their former housekeeper, Pearline, hoping she holds the solution to curing the undead. After an unsuccessful search for Pearline, Zach finds himself in a spiraling situation where he can’t comprehend the extent of the horror unfolding.
Ultimately, Zach returns to find Beth has transformed into a mindless, bloodthirsty zombie after devouring Maury, leading Geenie to restrain her. In a desperate bid, Zach convinces Geenie to let him take Beth, framing it as a hike. Outside, Kyle arrives with news that their parents are indeed alive and hiding out in a safe house. Armed with a gun, Kyle insists that Zach must let Beth go for their own safety.
During their grim hike, Zach reaches a cliff and candidly expresses regret for not fulfilling all of Beth’s desires during her living years. His heartfelt admission stirs a moment of clarity as Beth manages to mumble, “I love you too.” In a harrowing act of love and desperation, Zach shoots Beth, allowing her body to tumble down the cliff, the stove that bound her adding finality to their tragic narrative.
Zach ultimately reaffirms his bond with his surviving family, reuniting in the safe house alongside Erica, who is grappling with her own traumas from the undead outbreak. The chaos slowly gives way to hints of normalcy returning, as news reports suggest recovery is on the horizon.
With heartfelt resolve, Zach's mother takes him and Erica to Beth’s grave where he lays down Beth’s cherished scarf, a token of love and loss. As they leave, Zach finds the strength to ask Erica out for dinner, a glimmer of hope shining through the remnants of their tumultuous experiences.
