Haruji Masahiko Tsugawa is a shy, introverted young man who travels with his older, more outgoing brother Natsuhisa Yūjirō Ishihara for a sunlit summer by the sea. The brothers arrive by train to a friend’s seaside haven and quickly settle into the rhythms of vacation life, while Haruji’s thoughts keep circling back to a mysterious young woman he glimpsed at the station. They soon meet at their friend Frank’s clubhouse, an easygoing hub run by Masumi Okada, where Haruji’s fascination with the unnamed woman grows clearer as days pass.
Across the water, Haruji reconnects with the same woman, Eri Mie Kitahara, in Kamakura and the pair tentatively agrees to see each other again. Haruji spends time teaching Eri to water-ski, and their bond deepens through shared moments on the water and on the shore. The social circle holds a party where Haruji brings Eri along, and both brothers take turns dancing with her, until Haruji declines to stay and pulls away, leaving the impression of an fragile, evolving connection.
As the trio moves from daytime skimming of waves to nightlife scenes, Eri’s situation becomes clearer. At a nightclub, she is seen dancing with an older American man, then leaves him and agrees to meet Natsuhisa the next evening at a seaside club. During one revealing conversation by the water, Eri explains she is married, though her husband is largely absent because of work, and that she has affairs out of necessity to fill the void. Natsuhisa, drawn in by her honesty and vulnerability, begins a new affair with Eri and finds himself falling deeply in love.
The relationship tightens into a perilous triangle. Haruji and Eri share a moment on the beach, and the next morning he recounts the encounter, imagining plans to camp with Eri and her friends. Jealousy gnaws at Natsuhisa, who storms Eri’s home and spends the night with her until she tells him to leave. The next day, Natsuhisa reads a letter from Eri addressed to Haruji, hinting at an earlier arrival, and seizes the chance to reach her by taking Frank’s motorboat across the docks. Haruji borrows another motorboat in a bid to catch up.
The pursuit unfolds on the water as Natsuhisa and Eri sleep in one boat while Haruji searches through the night. Dawn reveals a tense standoff: Haruji locates the motorboats, circles aggressively, and when Eri attempts to swim toward him, he strikes. He hits Eri with the craft and, moments later, collides with Natsuhisa’s boat, killing him as well. The scene closes with Haruji driving away, unrepentant and alone, leaving the sea and its shattered relationships behind.
