Twelve-year-old Emily Windsnap is a girl who lives on the boat The King of the Sea with her mother Mary Penelope in Brightport Harbor. But she has never learned how to swim. Even though Emily begs for swimming lessons, her mom always says no. finally, Emily convinces her mother to let her take swim class at her new school. After a few shocking events, Emily discovers she is half mermaid, half human. Her legs become a tail when she is immersed in water and become legs again when she leaves it. However, her mother appears to remember nothing about mermaids or even Emily's own father. While exploring the ocean Emily meets another mermaid whose name is Shona, and they quickly become best friends. Together they discover that Emily's father is currently in the mer prison for marrying a human and that Mary is under a spell which makes her forget everything. The girls plan to save Emily's father, but do not know how. Later, Emily finds a chance to carry out her "daydream" plan and manages to drive their houseboat near the prison. Her mother follows and regains her memories of Jake Windsnap, her husband (Emily's father). They are caught by Neptune and brought to trial, where Emily manages to convince Neptune for the sake of their family's mutual love to let them go. Neptune releases the entire Windsnap family on one condition: they must live on a deserted island!