Irene and Wendy are sisters who live under the sea as magical creatures usually called Mermaids. Being descendants of Ocean's conquerors, they are given special privileges to visit earth as humans once in a while. Never once in Irene's wildest idea, she would save a stranger from being killed by the sea, and yes, her name is Kang Seulgi. It is infatuation at first sight for Irene which eventually turns into something more when she has her eyes landed on Seulgi. But the regulation is absolute and non-debatable that every human they have interacted with on land shall forget who they are once they got back to the sea. Seulgi has given her words to Irene that she shall stay as a loyal lover despite the long-distance relationship they have and yes, Seulgi has no idea who Irene really is. Irene visits Seulgi quarterly with different scenarios every time, simply because Seulgi couldn't remember who she is. Be it as a stripper, grave keeper, veterinarian, or even greengrocer, Irene doesn't mind if it means she can spend a week with Seulgi before saying goodbye temporarily. Time goes forward, Seulgi gets older while Irene stays the same. Irene definitely has to step up her game If she wants to keep Seulgi not only temporarily, but eternally.