事业爱情接连受挫的主人公·润（志尊淳）在某个绝望的夜晚，打算跳河自杀。

当他准备跨过桥栏的时候，突然听到了不可思议的庙会喧闹声。如被指引一般，润来到了一家钓金鱼的商店。

在那里，他得到了一条美丽的金鱼·琉金，并决定带回家饲养。而那条金鱼实则由人而变，并突然以人的姿态

出现在了润的眼前，润决定叫她琉（泷本美织）。琉正在找寻传说中的黑兰寿，且希望得到润的帮忙。

之后两人因为这奇妙的同居生活距离逐渐靠近，却由于润母亲·真由美（仙道敦子）的再婚对象--润的继父长坂（国村隼）的介入，让两人的命运走向了意想不到的方向……

One day, Ezawa Jun, who has trouble with work and in love, gets a ryukin from a goldfish scooping game at a festival. However, she is actually a beautiful girl and she suddenly turns into a human form and appears in front of Jun. Called Ryu, she is looking for the legendary black ranchu and asks Jun for his help. The two strangers begin living together and growing closer. Their fate goes in an unexpected direction.