Lena Whittaker, a girl living in the seaside town of Diamond Bay, California, suffers from strange symptoms as her sixteenth birthday approaches. She falls into fugue states where she hums the same tune, she feels like she's trying to find something even though she doesn't know what, and she finds herself drawn to the beach at Magic Crescent Cove. One morning, Lena sees a mermaid out in the cove. She decides to learn how to surf so she can look for the mermaid, even though her father doesn't want her to.
The Mermaid's Mirror contains examples of:
- Blackmail: Lena's friend Pem starts dating a college student named Max. Lena wants his surfboard, so she and his brother Henry threaten to rat on him for dating a fifteen-year-old if he doesn't lend it to her for two weeks.
- Calming Tea: Lena's mom responds to most problems by making a cup of hot, milky tea. Lena has to admit that she feels better after drinking it.
- Dramatic Drop: While snooping in her parents' room, Lena finds a newspaper clipping that says Lucy committed suicide. She drops the clipping in shock.
- Fainting: Lena's dad takes her to meet his boss, who works in a corner office on the twenty-ninth floor of a skyscraper. Being so high up makes Lena so dizzy that she faints. Her mermaid ancestry means that if she's too far away from the ocean, she suffers health problems.
- Fantasy-Forbidding Father: Lena has always wanted to learn how to surf, but her dad won't let her because he almost drowned in a surfing accident years ago. She has to take lessons in secret.
- First Kiss: Lena had hers with Kai, one of her best friends, a few months ago. Now they're dating. She's not really attracted to him, but she's glad she won't have to reach her sixteenth birthday without ever kissing a boy.
- Good Stepmother: Lena's father remarried when she was nine. Her stepmother, Allie, did such a good job of raising her that Lena thinks of her as her mom and almost never thinks about her biological mother, Lucy.
- Loose Floorboard Hiding Spot: In a corner of Lena's parents' room is an antique chair that is never used, and that usually has other things piled on top of it. Lena suspects that the chair is hiding something, so she pushes it aside and yanks up the carpet, under which the floor has been cut. Lena lifts the cut floorboards to find a sea chest full of pictures of Lucy, letters she wrote, news clippings about her "suicide," a jeweled comb, and the titular mirror.
- Magic Mirror: The titular mirror can show people their loved ones who are separated from them. After Lucy returned to the ocean, Lena's dad spent hours watching Melusina in the mirror as she went about her life as a mermaid, unable to remember anything about her life on land. Eventually he realized he was wasting his life, so he hid the mirror and almost everything else associated with Lucy. Years later Lena finds the mirror, sees Melusina, and realizes that the mermaid she saw in the cove is the same person as her vanished mother.
- Maligned Mixed Marriage: Mer society is quite xenophobic, and pains are taken to make sure merfolk learn as little about human society as possible. When Melusina fell in love with a human, her parents tried to feed her lotus blossoms to erase her memory of him, but she went on a hunger strike. Rather than watch her starve, her parents let her go. She donned the sealskin cloak that would allow her to survive above water and stayed on land until the full moon turned her tail into legs. But her father, Merrow, vowed that as punishment for taking his daughter, Melusina's husband could never go in the ocean again, or Merrow would give the order for sharks to eat him and his loved ones. Even after Melusina returned to the ocean, she was never considered the same as the other mermaids - because she had been "Riven," she had to sleep in a different cave than the others so that her dreams wouldn't contaminate anyone. When Melusina takes Lena down to meet her family, the other merfolk are accepting at first because they think Lena is going to live as a mermaid, but when Lena says she'd rather live on land, Merrow forbids her from contacting her mother ever again.
- Meaningful Rename: When Melusina went on land to live as a human, she started going by Lucy.
- Missing Mom: Lena has been told that Lucy died when she was four. She actually discovered the sealskin cloak that would allow her to transform back into a mermaid and returned to the sea, losing all her memories of her life as a human woman. The news reported that she committed suicide by jumping into the ocean.
- Parental Sexuality Squick: While Lena is searching her parents' room for the lock opened by the key the mermaid gave to her, she opens her mom's bedside drawer and is disgusted to find sexy underwear. She's afraid to open her dad's drawer because it might contain "marital things." She opens and closes it as quickly as possible, and doesn't describe what she sees.
- Poster-Gallery Bedroom: Lena's room is covered with pictures of waves and surfers.
- Sleepwalking: At the beginning of the book, Lena wakes up at the beach, having walked there in her sleep. Later, she dreams that a voice is calling her. She follows it out of her room, and wakes up halfway down the stairs.
- Too Unhappy to Be Hungry: After Lena goes into the ocean with Melusina, her father spends days sitting on the beach. His wife tries to coax him to eat, with little success. He also grows a Beard of Sorrow.
- You Are Grounded!: Lena tells her parents that she's seeing a movie with her friends, when she's actually surfing. When she gets home, her parents order her to her room and ground her for the next few days. Her dad picks her up after school because he doesn't trust her not to get into trouble while walking home.
