Yatsuse Hiiragi, a first-year high school student struggling with social acceptance and lacking close friends, meets Tsumugi, an Oni (demon) girl searching for her mother in the human world on a snowy summer day. Unlike Hiiragi, Tsumugi is unconcerned with others’ opinions and brings him along on her journey.
Tsumugi’s mother disappeared when she was young, leaving her with a good luck charm from a remote temple, her last known location. The journey begins at Hiiragi’s house, where his suppressed emotions cause snow to fall and attract “snow gods”. They escape and hitchhike with a young couple selling used clothing at street fairs. After helping the couple communicate better, they are given clothes for their journey.
On foot, Hiiragi’s emotions materialize as milky “Mini-Oni” bubbles, again attracting snow gods. They escape, but Tsumugi is injured, and Hiiragi carries her to an inn where she recovers while he helps with various tasks. The inn’s staff grows fond of them, but they soon depart.
In a nearby town, Tsumugi investigates an old painted screen depicting a conflict between snow gods and Oni, meeting an old man who runs a tea shop filled with travel mementos. His daughter and grandson give him purpose after his wife’s passing.
Reaching the temple, Tsumugi’s father reveals the story of her mother was untrue, causing Tsumugi to argue with Hiiragi. They part ways, and Hiiragi turns into an Oni due to his suppressed emotions. A snow god sweeps him up and takes him to the Oni village, just as his own father catches up but watches helplessly.
Hiiragi dreams of Tsumugi inside the snow god, causing it to disintegrate and release him. He is welcomed by the Oni. Meanwhile, Tsumugi and her father reach the Oni village through a secret tunnel. Despite trouble for leaving, Tsumugi finds Hiiragi in the village chief’s throne room. They reconcile, but Tsumugi locks Hiiragi in a pantry, insisting she must complete the final leg of the journey alone.
Tsumugi discovers her mother on magical Oni Island, controlling the snow gods with a magical mask. Her father frees Hiiragi, who confesses his affection for Tsumugi. He races across the snow to the island, facing snow gods along the way. At Oni Island, Tsumugi touches the mask, encountering her mother in a magical space. The mask breaks, allowing her mother to return but potentially unleashing the snow gods. Tsumugi and Hiiragi manage to control the fallout, vanquishing the snow gods, and Hiiragi turns human again. The villagers adapt to the new reality of an Oni village potentially visible, but with better weather. Tsumugi, her father, and her mother are finally reunited, and Hiiragi returns home, reconciling with his family and making friends. In a post-credit scene, Tsumugi visits Hiiragi by bus, renewing their friendship.