NOTE: This spoiler was submitted by Jeremy

The film opens like a fairy tale, with the Narrator (Julie Andrews) talking about a young woman named Marie-Josephe (Kaya Scodelario), who was born and raised in a convent. She is a rebellious free spirit who often causes trouble for the nuns, but mainly the Abbess (Rachel Griffiths).

In France, Marie-Josephe’s father, King Louis XIV (Pierce Brosnan), is preparing to give a speech to the French citizens, but an assassin fires his weapon at Louis. He is hit but unharmed as the man is apprehended. Fearing for his own mortality, Louis is told by Dr. Labarthe (Pablo Schreiber) that there is a special underwater creature who possesses magical powers that can grant him immortality. Louis sends for a group of fisherman, led by Captain Yves De La Croix (Benjamin Walker). They encounter the creature, a Mermaid (Fan Bingbing), and proceed to capture her with a net. Her mate is also caught and attempts to fight off the fishermen, but Yves simply lets him go back into the ocean before heading back to France.

Labarthe tells Louis that the procedure to make him immortal is accomplished during an eclipse and involves cutting out the Mermaid’s organ. The only one against this is Father Francoise La Chaise (William Hurt). Per the King’s request, La Chaise sends a message to the convent where Marie-Josephe is living, and the Abbess is eager to get rid of her. La Chaise travels to the convent to bring the girl back with a crew, giving her the impression that she is traveling to play music in Versailles. Marie-Josephe arrives in Versailles and is amazed by how beautiful the city looks. She befriends a servant named Magali (Crystal Clarke), but everyone else who doesn’t know she is the King’s daughter looks upon her as a common outsider.

The following morning, Louis meets with Jean-Michel Lintillac (Ben Lloyd-Hughes), son of a wealthy baron. Lintillac tells Louis that his father is gravely ill and ready to pass on. Louis discusses with La Chaise that Lintillac stands to inherit a fortune, and passing down his father’s title would benefit everyone.

While out for a stroll with Magali, Marie-Josephe notices others staring at her. They go to a fountain where she sees Yves for the first time and is smitten. When Louis and his men arrive, Marie-Josephe loses balance and falls into the fountain. Lintillac pulls her out, and she meets her father for the first time, though he still doesn’t tell her his relation to her. In the meantime, Marie-Josephe is set to practice music, since it is one of her skills.

The men bring the Mermaid into a chamber beneath the castle. Louis learns that she is despondent and gloomy after being separated from her mate, and he orders the men to get her well so that the procedure may go through without a hitch. Later, as Marie-Josephe tries to practice in her room, she senses a vibration against her harp and follows the noise she hears. It leads her to the chamber where the Mermaid is being kept. Marie-Josephe meets the Mermaid and is stunned by her. Yves comes by to catch her, but Marie-Josephe says that the Mermaid is making some kind of musical noise, which she syncs up with her violin playing.

The next day, Marie-Josephe plays music with the royal troupe before Magali helps her get ready for a ball later that afternoon. Everyone is in awe of Marie-Josephe’s beauty, and she shares a dance with Louis in front of everyone. She later finds him outside painting a picture of his late wife. He tells Marie-Josephe that she reminds him of her.

Louis appoints Lintillac with the title of Duke, and later holds a dinner in his honor. Yves and his crew discuss their tale of sailing to capture the Mermaid, to which Lintillac makes snide comments, prompting Yves to fire back with more comments that offend him. After the dinner, Marie-Josephe goes to visit the Mermaid again and goes underwater to be closer to her.

Yves later takes Marie-Josephe to ride horses, but she falls off and severely breaks her arm. Labarthe states that the arm will need to be amputated to prevent infection, even though everyone thinks there must be another way. Yves later carries Marie-Josephe along with Magali to see the Mermaid and places her in the water. The Mermaid uses her powers to heal Marie-Josephe’s arm completely. Yves and Magali are amazed at the Mermaid’s true power.

After seeing how Marie-Josephe’s arm has healed, she assumes that Louis has brought the Mermaid to France to heal the citizens from sicknesses, but she is told that the Mermaid is for the King’s purpose only. She later walks around the courtyard with Yves, and the two share a kiss.

Louis arranges for Lintillac to marry Marie-Josephe so that they can share in Lintillac’s fortune. La Chaise summons Marie-Josephe after she spends more time with the Mermaid, who took her swimming. Louis comes clean to Marie-Josephe about him being her father and then tells her that he wants her to marry Lintillac. She refuses and runs away crying, leaving Louis feeling remorseful.

La Chaise goes to visit the Mermaid and she communicates with him telepathically. Realizing she is a living creature with feelings, he confronts Labarthe about killing her to fulfill the King’s wishes, but Labarthe fully plans to go ahead regardless. La Chaise brings his complaints to Louis, who also refuses to change his mind. Meanwhile, Marie-Josephe goes to see the Mermaid herself, and she shows her a vision of Yves being the one to capture her.

Yves slips the picture of the procedure under Marie-Josephe’s door, alerting her to the King’s real plans for the Mermaid. She trashes the musical instruments and then confronts Louis over his plans. She says that she will marry Lintillac if he lets the mermaid go, but he refuses to honor her wishes. As everyone prepares for the wedding, La Chaise goes to Marie-Josephe’s room to quietly bring her to save the Mermaid.

The two go to the chamber where Yves is also trying to get the Mermaid out. Although Marie-Josephe reminds him of how he captured the Mermaid, he is committed to setting her free. They are found by Labarthe and another goon, but they fight them off to give the Mermaid time to escape. Labarthe shoots Yves, who falls into the water. The Mermaid pulls Yves with her as she swims out. Marie-Josephe gets Labarthe caught in a rotating wheel that pulls him underwater.

Marie-Josephe makes it to the side of a cliff where she sees Yves is okay. Louis and his men arrive and prepare to shoot the Mermaid to go through with the procedure, but Marie-Josephe gives Louis the choice to save his own life or his daughter’s. She jumps off the cliff and hits the water hard enough to injure her severely. The Mermaid grabs Marie-Josephe, and Louis has no choice but to tell his men to stand down with their rifles so that the Mermaid can revive Marie-Josephe. She and Yves swim to his boat while Louis watches them leave, and the Mermaid swims home. La Chaise tells Louis he is now a great king and a great man.

Marie-Josephe and Yves sail to find the city of Atlantis so that Marie-Josephe may reunite with the Mermaid, and she takes her underwater to see the city up close.

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King Louis XIV seeks out a Mermaid with magic powers that can grant him immortality. He calls for his secret daughter Marie-Josephe to arrive from the convent where she was raised to be present for the ceremony under the ruse of having her practice music in Versailles. When she gets there, she befriends a servant named Magali, falls in love with Captain Yves De La Croix, and becomes close with the Mermaid after learning she can communicate telepathically and also has healing powers.

Louis arranges with Dr. Labarthe to undergo a procedure during an eclipse so that they can cut the Mermaid's heart out and make him immortal. He also tries to get Marie-Josephe to marry a young Duke who is also a jerk. Marie-Josephe gets help from Yves and Father La Chaise to help free the Mermaid. They fight Labarthe and make it to the ocean so the Mermaid can swim free. Although Louis pursues, Marie-Josephe throws herself off a cliff so the Mermaid can revive her, and he decides to let them all go free.

Marie-Josephe and Yves travel together to find the Mermaid's home in Atlantis to visit her.