Valerie "Val" Nguyen is a prominent motivational speaker who harbors a deep-seated struggle with a compulsive urge to scratch the back of her head. This troubling behavior intensifies significantly after the tragic loss of her mother, Chi, in a devastating boating accident during Val's childhood.
As Val gears up for an international motivational speaking tour, she encounters an unexpected obstacle—she needs her birth certificate. This requirement prompts her to reconnect with her estranged family, leading her to her Aunt Thuy, who directs her towards her father, Sang. Once a soldier, Sang has now become a monk and is the keeper of the important family documents that Val seeks. However, the relationship between Val and Sang is strained, weighed down by the ghosts of their past, including Chi's untimely death and Sang's former battles with addiction.
During her emotional visit, Sang shares with Val a Vietnamese folk tale that centers around the Sanshi, a parasitic spirit known to prey on human hosts, spurring them into destructive behaviors. Val initially dismisses this tale as mere folklore until she stumbles upon old photographs depicting her mother with peculiar rashes, which sparks the unsettling possibility of a familial curse haunting them.
As Val's compulsions escalate, she inadvertently inflicts harm upon herself by scratching a wound on her scalp. This alarming revelation leads to conflict with her husband, Robbie, who discovers that Val has secretly been taking birth control pills, undermining their shared dream of starting a family. Seeking answers, Val confronts Sang, who divulges a dark secret—he inadvertently summoned the Sanshi during the Vietnam War, passing its curse onto Chi. In a shocking revelation, Sang reveals that Chi, in a desperate act to prevent the curse from spreading, nearly drowned Val as a child, forcing Sang to make a heartbreaking choice to save his daughter.
As Val's mental state spirals downward, she suffers a public breakdown during one of her speaking engagements and is subsequently hospitalized. Doctors alarmingly discover that she has scratched a hole into her skull, resulting in distressing hallucinations. Although Robbie brings Val home in hopes of a recovery, she remains tormented by the notion that the Sanshi is still lurking, refusing to let go.
In a gripping climax, Val attempts a perilous exorcism of the Sanshi by inflicting pain upon herself, believing that such suffering will draw the entity out. This act leads her into a fierce battle with a monstrous embodiment of the Sanshi. The struggle culminates in an intense moment where Val drags this spirit into a swimming pool, reminiscent of the fate Sang once tried to prevent. Robbie's timely intervention saves her, leading to a tense moment where Val emerges clutching a smaller, fetal-like manifestation of the Sanshi, symbolizing her confrontation with her own inner demons.
The film wraps up with Val revealing her pregnancy to Robbie, a moment filled with hope. Yet, the closing scene leaves the audience with a sense of unease—a small bug crawling on her baby introduces an unsettling possibility that the influence of the Sanshi may still linger, thus casting an ambiguous shadow over their future.
