
Zhang Yuqi as Ruolan ( 若兰), Xuan's business partner, the film's main antagonist.Lin Yun as Shan ( 珊), the film's titular character "the Mermaid".The screen then goes to Xuan and Shan swimming in the ocean along with the other merpeople. Xuan and Shan encourage the student to travel and explore the world. Xuan then introduces the student to his wife, "Lucy", who is actually Shan in a human disguise. The student asks Xuan whether merpeople are real and if the story that he fell in love with a mermaid is true, but Xuan denies it. Three years later, a student arrives at Xuan's home to thank him for setting up a scholarship for research into environmental protection. The police arrive and while Ruolan and George get arrested, Xuan collapses from his wounds. Shan insists that Xuan leave her, but he continues to carry her to safety.

As he is carrying her to the ocean, Ruolan angrily shoots him, but George stops her before she could injure him any further. Xuan, using a jetpack, arrives in time to retrieve a badly-injured Shan. As Shan attempts to escape, she is shot and forced onto land. The merpeople soon realize that the sonar had been turned off and return to the ocean. The merpeople are attacked by Ruolan's mercenaries, and many of them are killed. Xuan tries to stop them but they dismiss him and proceed to the abandoned ship. She is angered at Xuan's lack of affection and vows to kill Shan. Ruolan tells Xuan that George, her henchman, also believes in mermaids and has been trying to find one. He turns off the sonar but tells Ruolan that Shan is a mermaid and reveals where the other merpeople live. Xuan returns home and, after the shock has worn off, decides to investigate the consequences of his project. Octopus ( Show Lo), one of Shan's friends, gets mad and realizes that she has fallen in love with him. Xuan is about to be killed when Shan helps him escape. Against Shan's wishes, the others capture Xuan and explain that he is killing them with his land development project. There, he discovers that she and her family are merpeople. That night, as Shan gets into an argument with the other merpeople, Xuan goes to Shan's house to talk to her. Angered, Xuan demands Ruolan to cancel their business deal. Shan runs out, and as Xuan attempts to follow her, Ruolan stops him.

Xuan kisses her, but Ruolan interrupts them. A conflicted Shan rejects him and admits that she was sent there to kill him. Xuan admits that even though they only met the day before, he already knows that she is the one for him. The next day, Xuan takes Shan out on another date and proposes to her. When Xuan finally takes Shan home, Shan quickly sends Xuan away before her people can kill him. They spend the day together and develop feelings for each other. While there, Shan shows Xuan there are more important things than money, and Xuan is impressed with Shan's simple, amusing antics. Xuan decides to escort Shan back home, and along the way Shan takes him to a funfair where she works. Shan attempts to kill Xuan, but all her attempts backfire. Xuan, believing that Shan is an escort, calls her number in order to make Ruolan ( Zhang Yuqi), his business partner (and ex-girlfriend), jealous. At an extravagant party celebrating Xuan's business success, Shan, pretending to be a dancer, asks him to call her.

The merfolk send Shan ( Lin Yun), a beautiful young mermaid, who has been trained to walk on her fins and hide among humans, to seduce and kill Xuan. The few survivors live in an abandoned shipwreck in the gulf, and want to assassinate Xuan for his deeds. Unknown to him, the Green Gulf is the home of merpeople and the sonar (in addition to heavy pollution) has caused many of them to die or get sick. Playboy property tycoon Liu Xuan ( Deng Chao) purchases the Green Gulf, a coastal wildlife reserve, for a land reclamation project, and uses sonar technology to get rid of the sea life in the area.
