Canglan, a seventeen-year-old girl from the reef tribe, is the most "useless" existence in the deep sea - she has no poisonous spines that her tribe can use to defend themselves, and no ability to control ocean currents. She can only hear the whispers of corals. When her people were fighting each other for the "Heart of the Sea" (the core that maintains the energy balance of the deep sea), she was thrown into the eye of the storm as a sacrifice, but accidentally awakened the "Sea Guardian" Mo Yuan who had been sleeping for thousands of years.
Mo Yuan is the legendary ancient mermaid who can set off a world-destroying tsunami. His scales are as hard as obsidian, but he lost his memory when he woke up. He only became obsessed with the coral birthmark on Cang Lan's wrist. He said: "Your blood has the taste of the heart of the sea."
