In the lively waters of Southside Reef, a solitary bluestreak cleaner wrasse named Oscar pursues his lifelong ambition of becoming a tongue-scrubber at the renowned Whale Wash—a dream rooted in his childhood fantasies. Despite his aspirations, his peers cruelly mock him, leaving Oscar with a sense of deep inadequacy. Amidst his daily struggles, Oscar grapples with financial hardships, as he finds himself in significant debt to Sykes, the enigmatic pufferfish who runs the Whale Wash and serves as the right-hand man to Don Lino, the powerful mob boss.
Oscar's closest companion, the enchanting angelfish Angie, offers him a ray of hope by gifting him a precious pearl that he can pawn to alleviate his debts. Meanwhile, the relentless Don Lino, who leads a gang comprised of sharks, orcas, sailfish, and octopuses, grows increasingly frustrated with his vegetarian son Lenny, who lacks ambition. Don Lino assigns his eldest son, Frankie, the task of mentoring Lenny, hoping to instill a sense of purpose.
As Oscar tries to mend his finances by investing in a seahorse race, he unwittingly enters a realm of deceit. His encounter with the cunning lionfish Lola leads him to bet his entire saving on a questionable seahorse named "Lucky Day." While Sykes is displeased with Oscar's impulsive call, he secretly hopes for a change in fortune.
However, luck proves elusive as Lucky Day stumbles just before the finish line, sparking Sykes's fury. He orders two Jamaican jellyfish henchmen, Ernie and Bernie, to teach Oscar a lesson. Just as they're about to corner Oscar, Frankie appears, urging Lenny to devour him. Instead of succumbing to his brother's grim directive, Lenny shows mercy, freeing Oscar and advising him to escape.
The situation takes a tragic turn when Frankie meets a tragic demise after an anchor falls upon him, leaving Lenny drowning in grief. Overwhelmed with guilt and self-blame, Lenny decides to leave the reef, forcing Oscar to confront the remnants of his shattered life.
As Oscar's fabricated tale of slaying Frankie spreads throughout the reef, he basks in newfound fame and adoration from fellow sea creatures. Meanwhile, his manager, Sykes, conveniently forgives his debts, allowing Oscar to indulge in the trappings of his newfound celebrity status. In the backdrop, Lino dangerously escalates the search for his missing son Lenny, while unbeknownst to Oscar, Lenny is hiding from his father’s wrath, setting the stage for an unforeseen reunion.
Still wrestling with guilt over taking credit for Frankie's death, Oscar finds himself ensnared in his own web of lies. His relationship with Angie grows increasingly strained, culminating in her discovery of his deception, and she threatens to reveal the truth to the entire reef. In desperation, Oscar colludes with Lenny to stage a dramatic confrontation, further solidifying his reputation as the so-called "Sharkslayer." Lola, unaware of Oscar's real nature, lavishes attention on him, intensifying Angie's jealousy.
As tensions heighten, Oscar and Lenny entrench themselves into their separate lives. A confrontation with Angie reveals that her feelings for Oscar predate his fame, amplifying Oscar's regret as he navigates the fallout of his selfish choices. In a moment of emotional upheaval, Oscar ends his relationship with Lola.
Determined to reconcile, Oscar visits Angie at the Whale Wash, only to be confronted by Lino and Lola, who have crafted a plan to seek revenge for his betrayals. A climactic showdown unfolds as Oscar finds himself ensnared in deception. Just when it seems all hope is lost, Lenny intervenes, rescuing Angie and inadvertently exposing his true identity to Lino.
As the chaotic storm of events recedes, Oscar is forced to reckon with the consequences of his actions. Embracing newfound wisdom, he chooses to relinquish his wealth and luxurious lifestyle, opting instead to co-manage the Whale Wash with Lino's gang. The story culminates in a tender moment of reconciliation between Oscar and Angie, as they embark on a journey of love and unity within the vibrant community of the reef.
In a whimsical mid-credits scene, Lola unexpectedly arrives at Oscar's penthouse, only to encounter his quirky hermit crab friend, Crazy Joe, who has mysteriously made himself at home in the lavish setting.
