19-year-old Hinako Mukaimizu moves to a picturesque coastal town with dreams of college and a passion for surfing, not giving much thought to her future. However, her life takes a dramatic turn when a fireworks display inadvertently sets her apartment building ablaze. In her moment of need, she is rescued by Minato Hinageshi, a 21-year-old firefighter renowned for his unwavering sense of justice. Captivated by his formidable nature, Hinako finds herself drawn to him as they share time together, especially when Minato teaches her how to surf.
Minato, who has a special belief that the best surfing waves come after Christmas, chooses to go surfing alone the following day. Tragically, he attempts to save a jetskier and loses his life in the process. Overwhelmed by grief, Hinako opts to move away from the beach, trying to escape the memories. One fateful day, she discovers that every time she sings "Brand New Story," a song that holds significant meaning for both of them, Minato's spirit appears to her in the water. While no one else can see him, they resume their connection, spending cherished time together even in public.
Yet, Minato grapples with the painful realization of his situation. He becomes aware of his inability to physically be with Hinako, especially when his colleague Wasabi Kawamura makes his feelings for her known. In a heartfelt moment, Minato encourages Hinako to move forward with her life, a suggestion she initially rejects. As the story unfolds, Hinako ends up feeling too dependent on him, especially when she prevents his spirit from finding peace in the afterlife.
When visiting Minato’s home to pay her respects, she converses with his sister Yōko Hinageshi, who reveals the inspiration behind Minato’s choice to become a firefighter. In a surprising turn, Hinako learns that she had saved Minato from drowning in their childhood. Unlocking Minato's phone, she discovers a drafted message urging her to "ride her own wave." This revelation propels her to enroll in a lifeguard training course, a step she hopes will help her regain independence.
Meanwhile, Yōko explores her own relationship with Wasabi, who had motivated her to continue her studies after facing bullying. At work, Yōko overhears a plot by the same group responsible for the fireworks incident, as they prepare to repeat their actions near an abandoned building with a notable Christmas tree. Together, Hinako and Yōko track the group to gather evidence, but things take a dangerous turn as the fireworks ignite the structure, trapping them inside.
In a climactic moment, Hinako calls upon Minato’s spirit for help. Responding to her need, he conjures a wave of water to extinguish the flames, allowing them to escape safely on a backboard. After sharing poignant farewells with Hinako, Yōko, and Wasabi, Minato’s spirit ascends peacefully to heaven.
As Christmas approaches the following year, Hinako, Yōko, and Wasabi—who are now a couple—gather at Chiba Port Tower to celebrate Hinako’s achievement of lifeguard certification. While Yōko and Wasabi leave, Hinako sings to a fountain, but Minato does not materialize. She discovers a heartfelt message he had prepared for her the prior year inscribed on the tower, leading her to a moment of emotional release. Undeterred, she continues her commitment to lifeguarding and surfing, embodying the message of living her own life fully.
