For reference:

One summer night, Natsuki Uzume meets Yuki Iwato, an art student from the same university, and falls in love. After that, they spend night after night together. But as autumn approaches, Yuki suddenly tells him to find a cute girlfriend, wishes him well, and disappears from his life. Desperate to see her again, Natsuki visits her family home, but an unimaginable secret awaits him there: Yuki suffers from a mysterious illness that forces her to sleep through the winter each year. Is Natsuki willing to stay with her, even if it means spending every winter alone? And can Yuki, whose strange way of life has only brought her heartbreak, trust him enough to give him a chance?