Ji Yunhe possesses the rare ability to tame demons, but she is confined to the Demon Valley by strict imperial laws. Her life takes an unexpected turn when Chang Yi, a merman, is brought to her by a cruel princess. The princess orders Yunhe to give Chang Yi human speech and legs, forcing him to pledge loyalty. As Yunhe grows sympathetic toward the merman and falls for him, she longs to set him free. But helping him may come at a great cost, putting both their lives in danger.