A shy adolescent learns that she comes from a fabled royal family of legendary sea krakens and that her destiny lies in the depths of the waters, which is bigger than she could have ever imagined.
Fifteen-year-old Ruby Gillman learns that she is in the next legendary line of sea krakens. Despite her lofty destiny, she is desperate to fit in at Oceanside High. Ruby struggles even more to fit in when her mother forbids her from going to the beach. After disobeying her mother's rules, she discovers that she is descended from the warrior Kraken queens and will ascend to the throne as the Warrior Queen of the Seven Seas, her grandmother. The krakens are a race sworn to protect the world oceans from the vain, power-hungry mermaids by battling with eons. Ruby would need to embrace Chelsea, a mermaid-turned-human who enrolls at Oceanside High School.
Ruby Gillman is a 16 year old Teenager who's trying to fit in a human High School, and she's trying to get her mom to get her to Prom, well Ruby's Family is not allowed in the ocean ever for Reasons, well, the biggest secret is when she disobays her mother of going in to the Ocean, She finds out she's a descendant warrior of The Kraken Queens, and they are against the dangerous Sea creatures of them all, The Mermaids.—zaynrileykhan
In the town of Oceanside, 16-year-old Ruby Gillman and her family are secretly Krakens. Determined to fit in with humans, Ruby's mother Agatha forbids her from going near the ocean, including her school's prom at sea. Ruby's friends Margot, Trevin, and Bliss convince her to ask out her crush Connor anyway, but she accidentally knocks him into the water. Ruby jumps in and saves Connor - unknowingly triggering an underwater transformation - but new student Chelsea Van Der Zee takes the credit.
Noticing bio-luminescent suction cups on her fingers, Ruby hides in the school library but transforms into a Giant Kraken. She is spotted by Chelsea and by Agatha, whose ocean-dwelling brother Brill has been summoned by Ruby's transformation. Agatha calms her distressed daughter down to human size, and Ruby confronts her parents, who explain that the women in their family turn into powerful Giant Krakens while underwater.
With a reluctant Brill's help, Ruby sneaks into the ocean to meet her grandmother, the Warrior Queen of the Seven Seas, who reveals that Ruby is in line to rule as protector of the ocean. Other creatures have tried to seize power, including Leviathans, Umibozu, and most dangerous of all, Mermaids. The evil Mermaid Queen Nerissa found the Trident of Oceanus but was defeated by Agatha, who hid the Trident before abandoning her kingdom for a life on land.
Returning home, Ruby is attacked by vengeful sea captain Gordon Lighthouse, but is saved by Chelsea, who is actually a mermaid. Gordon's footage of Ruby in Kraken form goes viral, but Chelsea befriends Ruby, taking her on a "Super Sea Girl Ditch Day". Chelsea suggests that by finding the Trident, they can end the war between Krakens and Mermaids; her mother, Nerissa, was killed trying to recover the Trident from the Well of Seas.
Grandmamah trains her granddaughter to master her Kraken powers, unaware she is using them to practice entering the Well of Seas, and Ruby neglects her friends to spend time with Chelsea. Ruby's father Arthur, younger brother Sam, and Brill trick Gordon into "capturing" their pet sea creature Nessie, letting him believe he has ended the Kraken threat. Ruby tries to introduce Chelsea to her mother, but Agatha is unwilling to trust a Mermaid and forbids her daughter from returning to the ocean, but an angry Ruby transforms and storms off.
Agatha confronts Grandmamah, who explains that Nerissa never had a daughter, while Ruby retrieves the Trident but is betrayed by Chelsea, who is revealed to be Nerissa herself. Trapping Ruby underwater, Nerissa surfaces to use the Trident to destroy the Krakens, interrupting prom night as she fights off Agatha and Grandmamah. With encouragement from Brill, Ruby arrives to face Nerissa and saves the prom ship. Combining their powers, Ruby, her mother, and grandmother destroy the trident, and Nerissa is captured by Gordon.
Ruby's classmates hail her as their Kraken hero and she reunites with her family and friends while Connor accepts her invitation to prom, and Agatha and Grandmamah reconcile. Sometime later, Ruby and her family have embraced their identity as Krakens, while Ruby and Connor become a couple. Brill warns that a Devil Whale is heading toward the Kraken Kingdom, and Ruby leaps into action as the protector of the seas.
