The story begins with the voice of the Queen of Krakens (Jane Fonda), who uncovers a truth about the ocean that humans often misinterpret — that krakens are not the terrifying sea monsters they are believed to be, but rather formidable warriors of the deep. In the seaside town of Oceanside, we meet Ruby Gillman (Lana Condor), a young kraken living among humans with her family, including her mother Agatha (Toni Collette), father Arthur (Colman Domingo), younger brother Sam (Blue Chapman), and their beloved pet squid, Nessie. While Agatha is busy as a real estate agent, Arthur has a unique hobby of running an ASMR channel focused on creating ships in bottles.
Ruby has a tight-knit group of friends — Margot (Liza Koshy), Bliss (Ramona Young), and Trevin (Eduardo Franco) — and they are all excited about the upcoming school prom located on a boat. However, Ruby faces a hurdle: her overprotective mother has strictly forbidden her from venturing near the water, resulting in Ruby missing out on countless special moments.
Despite her mother's disapproval, Ruby desires to ask her crush, Connor (Jaboukie Young-White), to prom, believing that her heartfelt approach could win him over. Her friends encourage her with a confetti cannon for a memorable promposal. Yet, when Ruby attempts to use it, Connor inadvertently causes a messy accident that leads him to fall into the water. In a moment of bravery, Ruby jumps in to rescue him, only to discover that her entry into the ocean triggers a glowing purple transformation.
As Connor is pulled to safety, a crowd mistakenly attributes his rescue to the new girl in town, Chelsea Van Der Zee (Annie Murphy). As Ruby grapples with this misunderstood admiration, she feels her powers manifest uncontrollably, leading her to a towering transformation that culminates in chaos at the library.
When her colossal form draws the attention of her mother, Agatha, she rushes to help, accompanied by Uncle Brill (Sam Richardson), who has been dispatched by their grandmother, the Queen, with a mission to find Ruby, sensing the ocean's disturbance. Agatha reassures Ruby about her unique nature, urging her not to feel like a monster as they confront her emerging destiny.
As Ruby delves deeper into her lineage, she learns about the tumultuous history between krakens and mermaids, illuminated by her grandmother, who insists Ruby refer to her as "Grandmamah." However, upon being introduced as a successor among the kraken community, Ruby panics and flees.
In a twist of fate, Ruby falls victim to a hunter named Gordon Lighthouse (Will Forte), who has been fixated on capturing a kraken. Just when it seems he will succeed, Chelsea springs into action, revealing her true identity as a mermaid and believing she can help Ruby. Yet, Ruby remains conflicted, as her bond with she starts to neglect her friends, who grow concerned over her secretive behavior.
As the plot thickens, Ruby, aided by Chelsea, attempts to reclaim the powerful Trident from its hidden depths, leading to a dramatic revelation that Chelsea is actually the daughter of the mermaid queen, Narissa. As they collide in a fierce battle, Ruby fights to protect her friends and confront her own heritage, ultimately joining forces with both her mother and grandmother to vanquish Narissa and reclaim peace between their worlds.
In the film's moving climax, Ruby, having saved the prom-goers and revealed her true self, returns to her friends with a renewed spirit. She apologizes for her earlier secrecy, but her friends embrace her, even finding her more remarkable than before. Now, as the town embraces their kraken identity, Ruby and Connor navigate their budding romance amidst the acceptance of their vibrant, magical world. With new adventures on the horizon, Ruby dives once again into the deep, ready to explore the mysteries that lie beneath the surface.
