Mirai-chan, the newborn sister of four-year-old Kun, becomes the inadvertent catalyst for his jealousy as her arrival reshapes their family’s attention and routines. Her presence triggers Kun’s resistance to his new role as an older sibling, manifesting in emotional turmoil. A teenage Mirai from the future materializes in the family’s enchanted garden, leading Kun on time-travel journeys where he witnesses formative moments in their relatives’ lives: their mother’s childhood struggles, their great-grandfather’s wartime sacrifice, and their father’s unrealized architectural ambitions. These temporal excursions, orchestrated by the enigmatic older Mirai, illuminate recurring patterns in their family’s history—echoes of Kun’s own struggles mirrored in his parents’ pasts. As Kun confronts these generational parallels, his perspective shifts, culminating in a pivotal choice to shield infant Mirai from an imagined "Lonely Land" during a symbolic train station trial. This act marks his embrace of brotherhood. The future Mirai’s existence remains deliberately ambiguous, hovering between mystical intervention and childhood fantasy, yet her influence irreversibly alters Kun’s emotional trajectory. Her character operates as both guide and mirror, threading themes of cyclical legacy and sibling bonds through Kun’s transformation from resentful child to devoted protector.