The legend of "water monsters" is well known in the West. It is said that they are a group of beautiful girls living in the sea, river or forest. Cleveland (Paul Giamatti), a doorman of an apartment building, found a girl in the swimming pool while patrolling at night. He rescued her and learned that her name was Stoli (Bryce Dallas Howard). It turned out that she was the legendary water monster and was being chased by a monster. In order to help Stoli return to the original world, Cleveland asked many residents of the building for help, and slowly fell in love with this unique girl. Unfortunately, time is running out. The longer Stoli stays in this world, the more monsters will attack her, and even begin to threaten the lives of people in this world...