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 unemployed and searching for jobs. Among Aileen's new housemates is Lee Hyun-myung, Yoon Jin-ah's ex-boyfriend who is trying to win her back. With her new friends to help her, Aileen vows to find a way to win Shi-kyung's love, but in the end she falls in love with Hyun-myung. Although she initially disappears once her 100 days are up, she returns to life a year later to see her friends again.
The eighteenth mermaid princess of a mermaid kingdom in the sea. She finds a smartphone that fell into the sea and uses it to surf the internet and learn about the human world. She becomes infatuated with chef Kwon Shi-kyung; after saving him from the Han River, Aileen decides to become human so that she can be with him.
A human and resident of surplus house, he is unable to find a job with his art degree. At the request of his girlfriend Jin-ah, he gives up his pursuit of art and tries to find a desk job.
A resident of surplus house. She has a live webcast wherein she eats massive amounts of food for her viewers' pleasure. Hye-young becomes Aileen's friend and confidant when she discovers her true identity.
A resident of surplus house. He graduated from the law school of an elite university, then spent the next ten years failing the bar exams. He is addicted to video games, and quickly develops a crush on Ha-ni.
This drama airs on a cable channel/pay TV which normally has a relatively smaller audience compared to free-to-air TV/public broadcasters (KBS, SBS, MBC and EBS).
International broadcast
Malaysia - Under the title Idle Mermaid, it was aired in the Malaysia on TV9 from February 6, 2018 to February 21 on Monday to Thursday at 6:00pm Malaysia Time