Kathleen Conarn, an opera student living in Boston, suffers from mysterious chronic pain. Her whole life she's felt stabbing pain in her feet, and since she was sixteen she's felt bursts of pain in her mouth, like her tongue is being cut out. The only thing that really helps is being immersed in seawater. Kathleen starts hearing voices in the ocean telling her to "come home." When her girlfriend Harry Evans learns that Kathleen's mother, grandmother, and great-grandmother all committed suicide, she decides that she and Kathleen should go to County Clare, Ireland, where Kathleen was born, hoping that if they can find out why Kathleen's ancestors committed suicide, she might avoid the same fate. Kathleen learns that she is descended from Fand, also known as the Little Mermaid.
The Mermaid's Daughter contains examples of:
- Bath Suicide: Most of Kathleen's ancestors committed suicide in the ocean, with the exception of her grandmother Deirdre, who sliced her arms open in the bathtub. Her husband found her dead with no knife anywhere. The story of her death and the vanishing knife is a local legend.
- Beautiful Singing Voice: Fand had this gift. She passed it on to Kathleen's great-grandmother, Caolinn, who sang in pubs, as well as Kathleen herself, an acclaimed soprano whose voice amazes everyone who hears it.
- Child by Rape: Caolinn was a lesbian. She thought that meant the curse would end with her, until she was raped one night by a man who heard her singing. She named the resulting child Deirdre, meaning "child of sorrow."
- Childhood Friend Romance: Kathleen's parents, Robin and Moira, grew up in the same small Irish fishing village, fell in love at fifteen, and married at eighteen. Moira was only twenty-one when she died.
- Cope by Creating: Singing is one of the few things that can distract Kathleen from her pain. Her friend Tom is also an example - singing makes him forget about his insecurities and his father who disowned him for being gay.
- Curse Escape Clause: The cycle of pain and suicide can only end when a descendant of Fand uses the knife forged with Fand's sisters' hair to kill the person she loves so that when the person's blood falls upon her legs, she'll turn into a mermaid. Kathleen stars in an opera about the Little Mermaid, written by Harry and Robin, who change the ending so that the mermaid kills the prince and his bride and returns to the sea. Just before the climactic scene, Kathleen switches out the prop knife for the real knife. She doesn't kill her co-star, but she gets so far into character that he fears for his life. Theater has its own magic that is powerful enough that even though no real blood was spilled, Kathleen starts to transform into a mermaid as soon as the curtain drops.
- Dream Spying: After Kathleen is turned into a mermaid, the sea witches send dreams of her once a year to Robin and Harry so they know she's happy.
- Driven to Suicide: Kathleen comes from a long line of women who all committed suicide in their early twenties when their daughters were babies. They all suffered from the same pain she does.
- Hereditary Curse: Any mermaid in human form will inevitably feel horrible pain in her feet. Fand passed on her mermaid nature, and with it her curse, to all her land-dwelling descendants.
- I Want My Beloved to Be Happy: Before the mermaid Kathleen swims from Boston to Ireland, she tells Harry to write more operas and find another woman to love.
- Innocently Insensitive: During a master class, the world-famous soprano Ruzena tells Kathleen, "You are going to kill yourself. You will stab yourself, throw yourself into the sea." She's referring to the many tragic roles available to sopranos. She doesn't know Kathleen's mother drowned herself in the ocean when she was a baby.
- Locked Away in a Monastery: Caolinn and her lover, Marie, couldn't be open about their relationship in The '40s. They agreed that they would live together as spinsters and raise Caolinn's child together, until Marie's father forced her into a convent. They never saw each other again. Seventy years later, Kathleen and Harry meet Marie by chance at a pub, where Marie recognizes Kathleen's resemblance to Caolinn.
- Love at First Sight: Kathleen's great-great-great-grandmother, Muirin, grew up in England, where she fell in love with the servant of her suitor. As soon as she heard his voice and he saw her eyes, they were both entranced. He took her back to Inis Mór, the small Irish island he originally came from, and three more generations of mermaid descendants were born there.
- O.O.C. Is Serious Business: After Kathleen hears the voices saying "Come home," she doesn't want to go in the water anymore because she's afraid the voices will lead her to drown herself like Moira. Harry sees Kathleen stepping back to avoid getting her feet splashed and knows something's seriously wrong.
- Prince Charmless: Fand's prince was a cruel man who became sexually obsessed with her and planned to keep her on as a concubine. After Fand left him and went inland (instead of turning into a daughter of air as in the original story), he spent the next twenty years mistreating his family and the people in his court while calling his wife by the wrong name every night. When she finally returned, planning to kill him so she and her newborn daughter could turn into mermaids instead of living painful lives on land, the prince's first act was to try to rape her. Even after that, she still couldn't bring herself to kill him.
- Selkies and Wereseals: Kathleen meets a selkie on Inis Mór who draws her attention to the grave of three of her ancestors and tells her how she can contact the sea witches who transformed Fand. He proves he's telling the truth by pulling Kathleen overboard from the ferry and then turning into a seal in the water.
- Skinny Dipping: When Kathleen goes swimming while on vacation on Sanibel Island, she swims far enough out to pick her feet off the bottom, then takes all her clothes off. The more exposed she is to the water, the better she feels. Harry picks her clothes out of the surf for her.
- Switching P.O.V.: Kathleen and Harry narrate in the first person, and Robin narrates in the third person. The sea witches occasionally provide information about Kathleen's family history.
- Tomboyish Name: Harry is short for Harriet.
- Tongue Trauma: Fand gave up her tongue so she could take on human form. Her descendants have intact tongues, but they feel the same pain she did as part of the payment for being a mermaid who lives on land.
- Two-Person Pool Party: Kathleen and Harry have sex in the hot tub in their hotel on Sanibel Island.
- Underwater Kiss: After Kathleen is turned into a mermaid, she pulls Harry underwater for a series of passionate kisses. The two are about to have sex when Kathleen remembers that Harry still needs to breathe air.
