Molly, an overweight teenage girl, has always wanted to be a mermaid. She begs her mother for mermaid lessons for years and finally, on her thirteenth birthday, her mother gifts her a mermaid tail. Her mother is hopeful that the lessons will help Molly lose weight. She’s teased mercilessly by the other kids at school and has been told there are many activities she can’t do because of her weight. But when Molly goes to sign up for mermaid classes at the swim center, the instructor tells her there’s a weight limit and she cannot participate.  Crushed, Molly rides her bike along a trail to the river. She strips down to her swimsuit, puts on her mermaid tail, and topples into the river where she quickly realizes that she can’t swim. She’s about to drown when something grabs her by the waist and drags her to shore. When Molly comes to, she sees the creature that rescued her has a tail like hers but its scales extend all the way up its body. Molly realizes that the creature is a mermaid who lives in the river. She thanks the mermaid for saving her and the mermaid invites her to stay and keep her company. Molly decides to go home instead. At home, Molly’s mother asks about her mermaid lessons. Molly lies about the classes and pretends to enjoy the salad her parents serve her for dinner. They have her on a “diet” again, which she hates. She returns to the river the next day and asks Ametrine the mermaid to teach her how to swim while wearing her tail. Ametrine is funny and nonjudgmental, which is a relief to Molly.  Then, Molly’s mother finds out that she hasn’t been attending mermaid lessons. She tells her to stop splashing around in the dirty river and takes her tail away. Molly returns to the river and Ametrine offers to teach her how to be a real mermaid. She admits that the process will be painful, but Molly has already endured enough abuse from her parents and classmates. She agrees to the transformation. The town never finds Molly’s body, though she becomes more radiant in their memories. In the summer, some of the local children think they see two mermaids happily swimming together in the river.