Ningyo no Ashi to Hikikae ni

Synopsis

An unpopular man meets a cute young mermaid who comes onto land through a manhole. Tired of life in the sea, she says she wants to fall in love with a human so she can gain legs that will allow her to walk freely on land. Seduced by the mermaid's adorable smile, the man confesses, "I want to fall in love with you." A rollercoaster horror story that begins with a sudden romance with a mermaid. (Source: Number Nine, translated)