Aileen is a mermaid princess who is in love with Kwon Shi-kyung, a human celebrity chef. While recording a TV show next to the Han River, Shi-kyung hits his head and falls into the river, and Aileen stops him from drowning by kissing him underwater. Shi-kyung is pulled out from the water by Yoon Jin-ah, an intern who works for and is also in love with Shi-kyung. In order to meet Shi-kyung again, Aileen steals a magic potion from the male witch Ahn Ma-nyeo, and is transformed into a human girl. It's only after her transformation that Aileen learns that she has 100 days to find true love, or else she will die and turn into bubbles. Ahn Ma-nyeo helps Aileen by setting her up with the fake identity of Kim Ha-ni, and sends her to live in a house in Seoul with other young people, most of whom are...