It literally means " Sky," is a myth filled with gloom, memorable misdeeds, and dark secrets that are related to him more than he would like.
Yorito Morimiya is down right obsessed with the sky. So much that his room's ceiling is a big collage of the sky, made with photos he has taken. He is willing to ditch classes just to lay on the school's roof, watching the sky; he even forgets his sister Aono's birthday because in his mind, there's nothing but sky, sky, sky. He's even willing to wake up at 4AM just to take shots of the dawn sky. And that's how he meets Matsuri, a rather intriguing girl. From then on, Yorito is half-dragged to a story that spans centuries.