The movie begins with a heartbreaking scene where Bojana Novakovic depicts Hannah, giving birth to a daughter named Ava who tragically passes away shortly after birth. This devastating event leaves a deep emotional scar on the entire family, which includes her husband Ben McKenzie, and their two young sons, Steve and Kyle. In an effort to find some measure of peace and healing, Jack plans a family vacation to Japan. However, as tensions escalate in the Middle East, Jack, who works as a NATO AI operator, is called to stay at his post, leaving Hannah and the boys to travel alone.
Once in Japan, the family settles into an Airbnb in Tokyo. Their journey takes a turn when they venture out to a remote mountain town, where Kyle has a near-drowning experience in a lake. Fortunately, a local nurse and family friend named Iriko Miyu Yokota manages to save him. Although this traumatic incident sets a solemn tone, strange and unsettling events soon begin to unfold. Kyle starts displaying bizarre behaviors such as an unusual obsession with cucumbers, aggressive outbursts towards his older brother Steve, and episodes of vomiting green bile, which alarms Hannah and causes concern.
Hannah initially attributes Kyle’s odd behavior to trauma or stress, but Jack, watching from afar through video calls and messages, becomes suspicious of a more sinister, possibly supernatural cause. His research leads him to explore the legend of the Kappa—a Japanese water spirit reputed for possessing children and causing mischief or harm. As Kyle's condition rapidly deteriorates, Jack enlists the help of his friend Ryan Kane Kosugi, who becomes deeply involved in investigating the myth of the Kappa. Meanwhile, Hannah struggles with her mental health, which worsens with the use of prescription medication, leading to moments of doubt and fear.
The situation escalates when Ryan spots the creature and attempts to shoot it, only to be attacked instantly afterward. This attack confirms the supernatural threat that Kyle and the family are facing. The family's nightmare reaches a climax as Kyle's behavior becomes violently aggressive, culminating in him biting Steve, which prompts Jack to take drastic measures. Driven by desperation, Jack travels to Japan and confronts his possessed son in a tense and emotional showdown.
In a final act of sacrifice, Jack finds his wife dead and makes the brave choice to set the house on fire to free Kyle from the demon's grip. The ending scenes reveal that Jack faces trial, where his two sons—Steve and Kyle—are asked whether their father is a good dad. Despite the chaos and tragedy, the boys offer heartfelt testimonies that affirm Jack’s love and dedication to his family, providing a poignant conclusion to a story filled with supernatural terror, emotional pain, and parental sacrifices.
