Kasumi, a beloved and cheerful girl among her peers, conceals a unique secret—a butterfly-shaped birthmark adorning her ankle. Only Naoshi, her somber and isolated childhood companion, is privy to this hidden mark. Despite their shared past, the two have drifted apart over the years, with Kasumi yearning to rekindle their friendship. Yet, Naoshi keeps his distance, evading her attempts at reconciliation. What lies behind Naoshi's reluctance to reconnect with Kasumi remains a mystery waiting to be unraveled.