Two teenage best friends, Claire (Emma Roberts) and Hailey (JoJo), regret that they will be separated in five days, since Hailey has to move to Australia because of her mother's job. One night, after a violent storm, Hailey wishes for a miracle that will make her mother change her mind about moving, and so they find a mermaid named Aquamarine (Sara Paxton) in the swimming pool of their beach club. The girls are frightened at first, but then curious. They start to talk with Aquamarine and promise her they will be back in the morning.

The next morning, Claire wakes up and hears a commotion down at the swimming pool where they left Aquamarine. Claire's grandparents were clearing the pool because of the huge storm. Both Claire and Hailey quickly rush down to save Aquamarine but when they reach the pool , everything was already cleared. They hear noises by the nearby beach's snack shack and find Aquamarine, naked. The only difference is that she has legs instead of a mermaid's tail. Aquamarine tells them she can assume human form on land during the day if she does not get wet. Claire and Hailey become friends with Aquamarine, who tells them that she came to land to find love, an idea she knows nothing about, but desires desperately. She tells them that she made a deal with her father that if she can prove love is not a myth, she will not have to undergo an arranged marriage to a spoilt merman—but, she only has three days to do it. She notices a handsome lifeguard named Raymond (Jake McDorman) – whom Claire and Hailey have admired for a while – and enlists the help of the two girls to win his heart in return for granting their wish. The girls see their opportunity to keep Hailey from moving and quickly work on prepping Aquamarine on the how-tos of attracting a man. These self-educated relationship experts, having gained all their knowledge from reading magazines every day and often quote them, soon realize it's a lot easier said than done.

Claire and Hailey's rival a spoiled, rich girl, Cecilia (Arielle Kebbel) — tries her best to stop them, but fails. Also prominent is an eccentric, compassionate caretaker, Leonard, who rescues Aquamarine from public discovery. Eventually, it is not Raymond that convinces Aquamarine that humans can love, but Claire and Hailey, who are willing to give up their wish and their lives in the aiding of a friend. Raymond and Aquamarine share a kiss and promise to meet up again in the future, Raymond promises he'll wait for her. Hailey and Claire get their wish from Aquamarine, but decide not to use it until later. But it is mentioned that they plan to use it to make sure all three of them can visit each other every summer. It also occurs that Aquamarine uses starfish for earrings, and they give compliments to the wearer. Aquamarine gives both Hailey and Claire a starfish, telling Hailey she is beautiful inside and out, and Claire is courageous and has a true heart. Aquamarine also says that they can always call her on her shell phone.