Sweet and awkward 16-year-old Ruby Gillman (voiced in the original by Lana Condor) just wants to get along with the other kids at Oceanside High, but feels they’re turning their backs on her. She helps a classmate of hers (Jakoubie Young-White) with math, with whom she has a secret crush, but he only admires her for her facility with numbers. She can’t hang out with the popular kids at the beach because her overprotective mom (Toni Collette) categorically forbids her from diving in the water. When Ruby decides to disobey her and enters the sea, she discovers that she is descended from the great warrior queens of the Kraken and that she is destined to inherit the crown of her strong grandmother (Jane Fonda), the Warrior Queen of the Seven Seas. .
The Kraken swore to defend the seas from the greedy and haughty sirens, and the battle began long ago. However, Ruby will have to solve a serious problem: it happens that the newcomer to the institute: Chelsea (Annie Murphy) – also quickly becoming the most popular and prettiest in the institute – is actually a mermaid. The young Kraken will have to accept her lineage and gather her strength to save her friends and family.