主人公是为了调查人类生态而从土星来的外星人，他在地球上以真田家的次子・日出男的身份，与长子梦二、长女想乃和三儿子诗文一起生活。
伪装成他们的家人生活了23年，离开地球的日子还剩最后三天，日出男为作为人类还能做些什么而奋斗。

Kenmochi works as a novelist. He is able to recognize people who are suicidal. Kenmochi and his friend Urushihara meet these people and let them find hope in their lives.