Shy 16-year-old Ruby Gilman (Lana Condor) wants to fit in at Shoreline High, but she feels invisible to her classmates. She's tutoring her skateboard crush (Yabchi Young-White) in math, but he seems to just admire her academic performance, and her protective mother (Toni Collette) has banned her from going to the beach with the school's cool kids and has made it a strict rule that she'll never swim. But when she breaks her mom's number one taboo, Ruby discovers she's a direct descendant of the Siren Warrior Queen, destined to inherit her domineering grandmother (Jane Fonda), the Warrior Queen of the Seven Seas. The Siren race is sworn to defend the world and all its oceans, and has fought since ancient times against the vain, power-hungry mermaids. But Ruby encounters a serious and urgent problem: Chelsea (voice of Annie Murphy), the new transfer student at school, is not only beautiful and popular, but also happens to be a mermaid. If Ruby wants to protect the people she loves most, she must bravely accept her true identity and thrive.