Set in a near-future world where humans and merpeople live alongside one another, Chao follows Stephan, a well-mannered employee at a shipbuilding company with an idea for a propulsion system designed to be safer for marine life. However, his life takes an unexpected turn after an accident at sea brings him face-to-face with Chao, the princess of the mermaid kingdom.

Before long, Chao makes a surprising proposal: marriage. Their unlikely relationship soon captures widespread attention, with their union seen as a possible bridge between the human and mermaid worlds. As Stephan and Chao adjust to each other’s vastly different lives, their unexpected marriage slowly unfolds into a heartfelt and unconventional love story.