Lanyin, the princess of the merfolk who was cast into the human world, is adopted by the Wang family of fishermen and takes the assumed name Wang Zhu. Her tears can crystallize into mermaid pearls, but each pearl that forms shatters one of her protective heart scales. After discovering her secret, the Wang family repeatedly forces her to shed tears in exchange for wealth. Even after learning that doing so will harm her, they still sell her to a pearl merchant. Her adoptive younger brother, Wang Yaozu, risks injury to help her escape. In the depths of the living tide, Lanyin awakens her bloodline, reclaims the mermaid pearls taken from her by force, and brings an end to the war between humans and merfolk. When the tide gate seals off the two shores, she returns to the South Sea. The siblings remain separated by the sea, each living in peace.