Plot
Four children—Jerd, Sai, Noi and Ting—play hide-and-seek in the forest at night. Noi and Sai hide inside a house rumoured to be inhabited by the spirit of a Krasue that protects the forest. There, Sai encounters a krasue inside a wooden box.
Years later, Sai and Jerd remain in the countryside, while Noi is in Bangkok undergoing medical training. During World War II, all the professional nurses have left for Bangkok to care for the wounded, leaving teenagers such as Sai to help at the local hospital. Jerd has an unrequited crush on Sai, whose feelings for him are unreciprocated.
Sai reunites with Noi in the forest, rekindling their romance. However, Noi has returned with a tribe from his hometown to hunt a krasue. The tribe's leader explains that a female krasue can transform a human woman into one of their kind, while a male krasue, known as a krahang, experiences severe pain during the initial stages of transformation and risks having his intestines devoured by a krasue.
Sai begins noticing bloodstains on her bed that grow larger and darker each day. One night, she cries out in pain as her head detaches from her body and floats into the air: she has become a krasue. She ventures outside to hunt and feed before returning to her human form. Noi witnesses her transformation and helps her obtain meat to sustain herself, bringing them closer together. Consequently, Sai begins neglecting Jerd.
It is revealed that Jerd has known about Sai's condition all along and joined the hunting tribe to mislead its members. When the leader, himself a krahang, discovers Jerd's betrayal, he transforms him into a grotesque krahang. Sai visits Jerd, who begs her not to abandon him, as she plans to flee to Bangkok with Noi that evening.
Meanwhile, Sai's father takes her to a public film screening near the village's borders to disprove the rumours that she transforms into a krasue at night. Noi's monk mentor reveals an ancient legend about a krasue who, while married to a krahang, fell in love with a human and became pregnant by him. Enraged by her betrayal, the krahang killed both the krasue and her lover. According to the legend, every krahang is destined to fall in love with a krasue and eventually kill her by tearing out her heart, foreshadowing the possibility that Jerd will kill Sai.
During the screening, the villagers are horrified when Sai transforms into a krasue and pursue her with guns. An enraged Jerd confronts her, intending to kill her, but Noi persuades him to remember their friendship. Jerd relents, but the tribe's leader kills him before turning on Sai. The monk intervenes and kills the leader.
It is then revealed that Nual, the monk's wife, was the krasue who infected Sai. The monk had imprisoned Nual in a box in the forest in an attempt to prevent her death. Haunted by his guilt, he had helped Noi in the hope of sparing Sai the same fate.
Sai and Noi flee towards a river that leads to Bangkok, but the villagers have already destroyed Sai's body. Noi remains hopeful because her head is still intact. However, their hopes are shattered when Jerd's parents shoot Sai in the head, killing her.
