In a future society where humans and mermaids coexist, Stephan, a regular office worker at a shipbuilding company, is suddenly proposed to by Chao – a princess from the mermaid kingdom! With no time to make sense of what's happening, Stephan finds himself living together with Chao. As he is showered with Chao's pure and wholehearted love, Stephan gradually begins to fall for her. What will become of their unlikely romance?