During the Joseon era, in the island village of Nanyang, far from Hanyang. In this secluded coastal village, Cheong-ok meets Baeki, who is revered as the White Merman and kept confined in a temple for life. When Cheong-ok discovers the terrible secret the village elders have committed against Baeki, she tries to rescue him but fails, and her entire family suffers a brutal punishment. Fortunate to survive, Cheong-ok escapes from Nanyang to wait for the right moment, determined to avenge her family and save Baeki. Years later, as an adult, she returns to Nanyang and infiltrates the temple where Baeki lives, becoming the temple’s night attendant…