Set in 19th-century Vietnam during the Nguyễn dynasty, a remote village is gripped by a chilling mystery when a headless corpse is found and soon eight decapitated bodies surface in the lake, sparking a local belief in a supernatural guardian they call the water wraith.
Five years later, Moon Ngọc Diệp is distraught when her niece Nga vanishes. While the villagers insist the water wraith took Nga, Moon refuses to accept that explanation and seeks help from Detective Kiên Tín Nguyễn, who years earlier had arrested her corrupt husband. The investigation begins with Kiên arriving at Nga’s home, where a thief broke in the night Nga disappeared and shattered a ceramic plate. At the lake, a villager finds Nga’s shoe with seeds from the xanthium plant, and a visit to the village shaman confirms that xanthium grows near the waterfall close to the village.
Moon and Kiên trek to the waterfall and find traces of blood. Moon then shares her painful past: Nga’s biological mother—Moon’s sister—had abandoned Moon when she was a baby. Nga grew up believing Moon was her true mother. Nga’s father, Vinh, disliked their closeness and forced Moon to leave. Moon disappeared, Nga waited for her return for years, and when Nga grew up, Moon came back. Nga forgave Moon and welcomed her back into her life.
Back in the village, Kiên experiences visions of Nga being taken by the water wraith, her head severed. He questions Tuyet, the district chief’s daughter, who was the last person Nga interacted with before vanishing, and discovers a connection between the broken ceramic plate and Thac, Tuyet’s fiancé. Investigating Thac’s house, Kiên learns that Thac and Nga were in love, but Thac could not marry Nga because he was already engaged to Tuyet. During a family meeting, Thac proposes making Nga his second wife, a notion Tuyet’s family rejects. Later, Tuyet’s family accuses Nga of pregnancy, a claim Thac denies.
To trap the thief, Moon hosts a knife-throwing contest to lure the culprit out. The suspect is identified as a man named Dong. During interrogation, Dong accuses Vinh of having an affair with Vuong, the District Chief’s wife. Kiên interrogates Vinh, who confesses that he struck a deal with Vuong to separate Thac and Nga in exchange for money to pay his debts. After this, Kiên arrests Tuyet, suspected of harming Nga, and Tuyet admits she accidentally struck Nga’s head near the waterfall. As the investigation unfolds, Kiên continues to see visions of the water wraith.
Vuong orders her servant Mui to ensure Dong kills Nga. A flashback reveals that a week earlier Tuyet struck Nga at the waterfall; Mui followed, found Nga unconscious, and informed Dong, who seized Nga’s body and imprisoned her in a secret cave, using her to blackmail Vuong. Not long after, a woman’s corpse believed to be Nga is found near the lake. Consumed by guilt, Vinh hangs himself. Vuong, eager to see Nga dead quickly, orchestrates Dong’s plan to kill Nga in exchange for the hidden payment. Dong kills Mui, believing she betrayed him, and drags Vuong to his place. Nga escapes into the forest with Moon, knocking Vuong unconscious and tying her up.
Nga, Moon, and Kiên return to the village, where Thac welcomes Nga back. As Kiên prepares to depart, he reveals the true source of the village’s haunting: a plant used by the village shaman to induce frequent nosebleeds and hallucinations, making people see apparitions of the water wraith. Kiên leads his men to arrest the shaman, and they uncover the District Chief’s severed head in the shaman’s house. Guided by a feather from the shaman’s winter cloak, Kiên and his men pursue the culprit north, bringing the long mystery to a startling turn and a difficult reckoning for the village.
