Amidst the glimmering sunrise over the ocean, sailors awaken filled with excitement, convinced they have discovered a mythical mermaid. In their fervor, they brandish their spears, inadvertently endangering a frightened dolphin. Boldly, Prince Eric ([Jonah Hauer-King)] leaps from his ship to save the innocent creature, expertly disarming the sailors in the process. His devoted aide, Grimsby (Art Malik), stands by protectively, tempering Eric’s adventurous nature. In this chaotic moment, a telescope is accidentally sent plummeting into the water.
Beneath the waves awaits a magical underwater world, teeming with life. King Triton ([Javier Bardem)] gathers his seven daughters — Perla ([Lorena Andrea)], Indira (Simone Ashley), Mala (Karolina Conchet), Tamika (Sienna King), Karina ([Kajsa Mohammar)], Caspia (Nathalie Sorrell), and the spirited Ariel ([Halle Bailey)] — as they prepare for the highly anticipated Coral Moon festival. Concerned for Ariel's well-being, Triton dispatches his loyal crab advisor, Sebastian ([Daveed Diggs)], to keep a watchful eye on her.
During their thrilling escapade, Ariel and her faithful companion Flounder ([Jacob Tremblay)] stumble across a shipwreck. They discover a peculiar fork, which they humorously mistake for treasure, only to be confronted by a lurking shark. In a display of quick thinking, Ariel uses the reflection to distract the predator, enabling their narrow escape.
As they navigate their vibrant underwater domain, Ariel and Flounder encounter Scuttle (Awkwafina), an eccentric bird who claims to be an authority on human culture. With humorous flair, Scuttle refers to the fork as a “dinglehopper,” insisting it’s a grooming tool for humans. Shortly thereafter, Sebastian rushes in to announce that Ariel has missed the critical Coral Moon celebration and insists they must return to their father. Meanwhile, the treacherous eels, Flotsam and Jetsam, lurk in the shadows, serving Ursula ([Melissa McCarthy)], Triton’s malevolent sister, who is plotting to ensnare Ariel and usurp her brother's throne.
When Triton confronts Ariel regarding her fascination with the human world, it sets off an emotional rift, leaving Ariel feeling despondent. Retreating to her private collection of human artifacts, she expresses her deep desire to experience life above the waves alongside Flounder, yearning to be “Part of Your World.” Disregarding Sebastian's cautious advice, she feels an irresistible pull to join Eric's birthday festivities, while Grimsby attempts to keep the prince focused on his royal obligations.
As the celebration unfolds, Ariel watches from the shadows, unseen by Eric, though his dog Max somehow senses her presence. Suddenly, a violent storm descends, wreaking havoc on the ship. In a display of heroism, Ariel dives into the tempestuous waters to rescue Eric from drowning, bringing him safely to shore, but the chance to reveal herself is thwarted by Grimsby.
Back in the underwater kingdom, Sebastian endeavors to coax Ariel home with an upbeat tune celebrating life beneath the waves (“Under The Sea”). Yet, Ariel becomes captivated by a cherished statue of Eric, intensifying her longing for the human realm. Struck by her bravery, Eric resolves to seek out the mystery girl who saved him, convinced she is his destined love.
As Ariel and her sisters begin to clean up after the shipwreck, tensions escalate due to Triton's visible disdain for humans. Sebastian alerts Triton to Ariel’s feelings for Eric, igniting a fierce confrontation. In a moment of rage, Triton destroys Ariel's cherished human artifacts, leaving her heartbroken.
Meanwhile, Ursula concocts a sinister plan, luring Ariel into danger. Unaware of the looming threat, Sebastian and Flounder closely follow her. When Ariel meets the cunning Ursula, she unwittingly falls into the sea witch's trap regarding the human world. Ursula boasts of her exceptional powers and offers Ariel a tantalizing deal: her voice in exchange for legs, with the stipulation that she must procure Eric’s kiss within three days.
Once on land, Ariel boards a ship bound for the castle, where she meets Eric. Struggling to reveal her true identity amidst his warm welcome, she feels the weight of her situation, while Sebastian’s apprehension grows regarding the potential consequences of Ursula’s contract.
Determined to reclaim her voice and identity, Ariel sneaks into Eric’s library, captivated as he recounts tales of adventure, solidifying their bond while she grapples with her emotions and the implications of Ursula’s deal. The next day, Eric persuades a hesitant Grimsby to accompany Ariel on a delightful outing. During an enchanting carriage ride through a lively marketplace, Sebastian, Flounder, and Scuttle devise a plan to ignite their romance, culminating in a picturesque boat ride complemented by nature’s sweet melodies (“Kiss The Girl”). Unfortunately, this magical moment is interrupted by the scheming Flotsam and Jetsam.
Under the sea, Triton becomes increasingly worried about Ariel's absence, reflecting on his past decisions. Suddenly, Scuttle bursts in with shocking news of Eric's imminent engagement. Ariel’s heart shatters as she witnesses Eric with Vanessa ([Jessica Alexander)], who has deceitfully taken her voice, prompting her to flee with her friends rallying in support.
Creating a clever diversion, Scuttle infiltrates Vanessa's domain, exposing Ursula's true identity just as Ariel prepares to act. Flounder rushes to notify Triton. As the engagement ceremony commences, chaos erupts when Scuttle causes a commotion, allowing Max to rescue Ariel from Ursula’s grasp. In a climactic confrontation, Ariel shatters the enchanted locket containing her voice just as her deadline nears.
Ursula reveals her monstrous form, dragging Ariel into the depths of the ocean while Eric remains stranded on the shore. Triton confronts the wicked sea witch, ready to defend his daughter. In a moment of bravery, he relinquishes his trident, granting Ursula extraordinary power.
Courageously, Eric ventures into Ursula's lair, leading to an epic battle that unites him and Ariel against the malevolent witch and her treacherous minions. Embracing her newfound strength, Ariel stands firm against the eels, resisting Ursula’s horrifying transformation. In a daring final move, Ariel commandeers an abandoned ship, wielding it against Ursula, whose downfall comes at the jagged edge of the weapon.
With harmony restored, Eric emerges from the ocean, his heart heavy yet steadfast. Surrounded by those he loves, he acknowledges his true feelings for Ariel. Understanding her dreams, Triton makes a pivotal decision, restoring Ariel to her human form.
Overjoyed, Ariel and Eric share a profound kiss, marking the beginning of their new adventure together. Bound by love, they sail toward the promising horizon, embracing the wonders of their interconnected worlds alongside their friends — Flounder, Sebastian, Scuttle, Triton, and her sisters — as they embark on a hopeful and harmonious future.
