In 1693, natural scientist Yves de la Croix captures two sea monsters, a live female and a dead male, and transports them to Versailles for study. His younger sister Marie-Josèphe, lady-in-waiting to King Louis XIV's niece, assists him by feeding and training the live monster. She gradually realizes that the monster is not a mindless beast, but a sentient being.
The Moon and the Sun contains examples of:
- Bathtub Mermaid: Yves keeps the sea monster in the Fountain of Apollo, around which he has a tent and a cage constructed.
- Child by Rape: Haleed tells Marie-Josèphe that they are actually half-sisters. Haleed was conceived when Marie-Josèphe's father raped a slave who had been brought from Turkey.
- Fluffy Tamer: Marie-Josèphe sees herself as this at first. Yves thinks the sea monster is incapable of learning or understanding anything, but Marie-Josèphe trains her to eat fish from her hand, do tricks for an audience, and repeat "Fish" like a parrot. When Marie-Josèphe realizes that the sea woman is actually an intelligent being, others accuse her of getting overly attached to a pet.
- Force Feeding: During the voyage from the sea monster's Caribbean home to France, she refused to eat, until Yves force fed her dead fish. At Versailles, Marie-Josèphe feeds her live fish, which she eats willingly.
- The Grand Hunt: Louis releases some animals from his menagerie, including camels, so he and his court can hunt them down.
- Harmful Healing: Marie-Josèphe blames this for her father's death. She thinks he would have recovered from his illness if the doctors hadn't bled him - after they did, he just got worse and worse. In the present day, the King's doctors try to cure her of her "delusions" about the sea woman by pinning her down and cutting her arm while she struggles. The wound becomes infected.
- Her Boyfriend's Jacket: Marie-Josèphe wonders about the light green lock in the sea monster's dark green hair. It turns out to be a token from her lover, the dead sea monster, who let her cut off some of his hair and tie it into her own.
- Humans Are Ugly: The sea woman thinks humans are ugly because of their smooth faces, which remind her of eels.
- If I Wanted You Dead...: At first Marie-Josèphe is afraid the sea monster will hurt her, until she drags her into the fountain and then lets her climb out unharmed. Marie-Josèphe thinks that if the monster wanted to kill her, she would have done so right away.
- Immortality Seeker: This is Louis' main interest in the sea monster. He hopes Yves' dissection will find an organ that confers immortality.
- Ironic Name: Lucien Barenton, Count de Chrétien, the King's advisor, is an atheist. Several characters comment on the irony of his name being Chrétien.
- Locked Away in a Monastery: After Marie-Josèphe's parents died, Yves left her for five years in a convent, where she was miserable. She was forbidden from listening to or composing music or reading about the natural world, and was punished harshly for asking the wrong questions.
- Mermaid Problem: Sea monsters have two tails, with human-like genitals between them. At one point the monster shocks a crowd of spectators by flashing them.
- No Periods, Period: Averted. Haleed and Marie-Josèphe both get their periods. When the sea woman smells the blood on Marie-Josèphe, she sings a warning, worried that the tigers in the King's menagerie will smell it too, like sharks, and kill her.
- Significant Name Shift: Marie-Josèphe originally thinks of the captive as the sea monster. Once she realizes that she's actually sentient, she starts thinking of her as the sea woman. Later she dubs her Sherzad, after her mispronunciation of "Scheherazade."
- Slave Liberation: Marie-Josèphe's family has a Turkish slave named Odelette, who Marie-Josèphe always assumed enjoyed Happiness in Slavery. When Odelette tells Marie-Josèphe how horrible it actually is to be a slave, and how much she wants to live in her mother's homeland, Marie-Josèphe tells her that she is unofficially free, even though she won't be legally free until she can persuade Yves to sign the manumission documents, and that she will adopt her as her sister. Odelette changes her name to Haleed and becomes a practicing Muslim. In the end, Yves signs the documents, and the King gives Haleed some pearls that she can use to travel to Turkey.
- Stay in the Kitchen: When Pope Innocent XII visits Versailles, he criticizes Marie-Josèphe for composing music because he believes that women should be silent.
- Unscaled Merfolk: Sea monsters have thick, leathery skin with no scales. Yves says that they're more like dugongs than fish.
- Wound Licking: Sea folk lick their wounds to heal them. After Marie-Josèphe is bled, the sea woman licks the infected wound on her arm, which heals it within hours. Marie-Josèphe doesn't make the connection; later, when the sea woman injures her shoulder in an escape attempt, Marie-Josèphe scolds her not to lick it, thinking it will make it worse.
- Zipping Up the Bodybag: After Yves becomes convinced of the sea woman's humanity, he drapes a cloth over the dissected sea man's face and wraps him in a shroud.
