Kun Oota grew up with all the privileges of a tranquil city household, complete with his devoted dog, Yukko, and affectionate parents. But the birth of his baby sister, Mirai, marked a sharp turn in his life as he found himself no longer the sole center of his parents' world. Kun tried to cope with his frustration, throwing tantrums and causing commotions, yet all the while observing how his parents showered his younger sibling with love and care. To adapt to this overwhelming change, Kun sets out on a thrilling journey through time, exploring the past, present, and future of his family. Along the way, he learns the true meaning of family and the responsibilities of being an older sibling.