The summer after college, a young woman and her family go on their usual vacation by the beach, only for her to accidentally drown while swimming alone. As her soul leaves her body, she is given a choice - death, or a second life as one of the Mer, humanoids with fins and the ability to breathe water. She chooses the latter, renames herself Val, and is adopted by a clan of fellow Mer.
The Mer contains examples of:
- The Ageless: The Mer age a few years per century up to a certain point, and then stop. They never grow old - they can only be killed by illness or injury, and they rarely get sick.
- Amazing Technicolor Population: Mers come in every skin color, which is usually completely different from the one they had in their first life. Val is lavender, Shona is green, Will is blue, and Liam is red.
- The Atoner: At the end of the book, Liam leaves the clan to travel alone, trying to atone for emotionally abusing Hazel and drowning Val. He doesn't know if Water will ever forgive him, but he wants to try.
- Babies Ever After: At the end of the book, after Val and Will have become a couple, they adopt an infant who drowned in a boating accident. Mer age so slowly that raising him will be a millennia-long commitment.
- The Bermuda Triangle: The Big Gathering of Mer is held in a different place every year, but every few years they hold it in the Bermuda Triangle. A lot of people have drowned there, and humans tend to avoid it, so a large number of Mers have made their home there.
- Betty and Veronica: Will is a kind, helpful, and reliable Mer whom Val instinctively likes and trusts and who quickly becomes her closest friend among the Mer. Liam is incredibly handsome and charming and instantly sweeps Val off her feet, but she soon realizes that his charm is a facade, although she can't tell what he's hiding. Their romantic histories fit the trope, too - Will has never been in a relationship since his transformation, while Liam has had a number of acrimonious breakups. Val's attraction to Will develops slowly, and in the meantime she's annoyed with herself for being so attracted to someone as obviously untrustworthy as Liam.
- Character Tics: Val has a lifelong habit of tousling her hair with her hands.
- Fell Asleep Crying: Val cries herself to sleep during her first night as a Mer, her tears dissolving into the water.
- The Lost Lenore: Liam used to be engaged to a woman named Margaret, but he left her to fight in The American Civil War, and then was knocked into the water during a naval battle, where he became a Mer. He later tried to reunite with Margaret, in defiance of Mer law, but she fled in horror from the sight of his transformed body. He has spent the last 150 years pining away for her.
- Loving a Shadow: Since his transformation, Liam has fallen for several women who he thought could take Margaret's place, only to mistreat and reject them when they failed to live up to his idealized memory of her. Of all the women he did it to, Hazel took it the hardest - she spent years thinking of herself as defective because Liam never let her forget that she wasn't as good as Margaret.
- Meaningful Rename: All Mer are strongly encouraged to take a new name for themselves after their transformation to emphasize the deaths of their human selves. Shona's name means "God is gracious" because she's grateful to Him for giving her a second chance at life. Will named himself after the only thing he had left after the loss of everything from his former life. Val, originally named Elizabeth, gave herself a name short for Valor.
- Military Brat: Will's father was in the navy. He was lonely as a child because his father was always deployed, his mother was dead, and his brother was at military school. He eventually came to see his transformation into a Mer as a good thing, because the clan became his family.
- Parental Substitute: Shona is the oldest Mer in the clan, and has taken many younger Mer under her wing. Some of them call her "Ma."
- Schedule Fanatic: As a human, Val carried her day planner everywhere and liked to have every minute of her day planned out. The lack of a schedule is one of the things she finds hard to deal with as a Mer.
- Spit Take: When Shona tells Val that she became a Mer centuries ago, Val is so startled that she chokes on ocean water and sprays it back out.
- Taking the Heat: Shona confesses to killing Hazel, Ella, and Val in order to protect Liam from being punished for his crimes. Will believes her and is devastated to learn that his adopted mother is a murderer, but Val quickly realizes she's lying.
- They Would Cut You Up: Mer are strictly forbidden from trying to contact anyone from their former lives because if the humans find out that they exist, they might be dissected, experimented on, or subjected to Superhuman Trafficking.
- This Is Reality: When Val and Will are arguing about who drowned Val, Will says, "It's always the person you least suspect." Val says, "That's not even true! In reality, it's usually the person you do suspect! This isn't a fantasy, it's real life - oh, I can't even say this! I'm purple, for God's sake! I've got gills! It doesn't matter if we look unreal, we are real, real people with real emotions."
- Threatening Shark: Val and Will come back from a trip to find the Clan House being attacked by a shark. The creature rams into the building a few times, then swims away. Shona says the attack was a warning from Water, the spirit worshiped by the Mer who may be the same entity as the Christian God. Water normally protects the Mer by keeping predators away; if He meant to punish them, someone would have gotten hurt, but instead the only thing damaged was their home.
- Too Unhappy to Be Hungry: Val swims back to the beach where she drowned to catch one last glimpse of her family, where she overhears her brother saying that her parents haven't eaten since she went missing two days ago.
- Underwater Ruins: One large Mer settlement is in the remains of an Aztec city on an island that sank after an earthquake and tsunami.
