Fourteen-year-old Claire Morimura moves to a coastal town to live with her uncle after her mother departs for Paris to work as a hat designer. Claire carries a deep, unresolved grief shaped by her father's vanishing at sea a decade earlier during a yacht tour; she clings to a steadfast belief in his survival and eventual return. This emotional burden intensifies with the recent death of her pet dog Papi, a farewell gift from her mother, plunging her into profound isolation. Her withdrawn state persists until she encounters Spank, a stray dog whose owner disappeared under similar maritime circumstances. Adopting Spank sparks a transformative bond. His lively, often troublesome antics disrupt Claire's melancholy, offering vital companionship and emotional stability.

Starting at a new school after summer, Claire navigates adolescent life: she develops a crush on Andreas, the sporty brother of Spank's missing former owner. Her school interactions include facing persistent teasing from local boy Seiya – his mocking nicknames like "Seaweed Hair" mask an underlying attraction. She also meets Serino, a wealthy classmate whose initially aloof demeanor hides a fundamentally decent nature, reminiscent of the complex personality of Serino's cat, which becomes an object of Spank's affections. Claire's journey centers on resilience, processing parental absence, overcoming loss through her bond with Spank, and forging new connections, all while her conviction about her father's fate remains a constant backdrop to her daily life and growth.