Troy, Donal Logue, is revealed to be alive and held captive by the marines, locked in a cage where he endures painful vaccines. A tiny con artist shark named Ronny, Jamie Kennedy, helps Troy escape and pledges revenge on Pi, the reef’s steadfast guardian. The news of Troy’s return ripples through the underwater community as the danger suddenly seems personal, not merely territorial, and the stakes grow with each passing day.
At the reef, Pi, Drake Bell, lives as a devoted father, finding happiness in a world that has no sharks circling above the coral. Yet the mood shifts when Troy and his band of marauders arrive at the reef wall, threatening the fish with a grim forecast: in four days the water will rise high enough for sharks to swim over the barrier. Nerissa, Rob Schneider, the old turtle, urges Pi to defend the reef but also tries to spark a new spirit among the other fish. He tests their limits while Pi grapples with the idea that leadership may require more than traditional training. Cordelia, Busy Philipps, a bright and brave presence on the reef, supports the community and becomes entangled in Ronny’s fragile allegiance.
As Troy presses forward, he forces Ronny to follow his orders by dangling a grim bargain: loyalty in exchange for freedom. Ronny disguises himself with kelp and makeshift dentures, and Troy’s threats push him to accept a dangerous role. Observing Pi’s flawed training, Ronny proposes a plan that could unite the reef: an underwater extravaganza where every fish showcases a unique skill, hoping to lure humans to defend them from the looming threat. The idea excites some and unsettles others, and Nerissa hints that Ronny may offer something Pi hasn’t considered—a chance for the reef to prove itself, not merely replicate what Pi has done before. A vision of the legendary Sea Dragon surfaces in Nerissa’s eyes, signaling a test of worthiness for those who would command its power.
With this plan in mind, Pi asks for resources to craft a shark trap designed to catapult Troy out of the sea. He and his son, Junior, Frankie Jonas, set the trap in motion while Bart and Eddie guard the cave. Ronny, still torn between loyalty and survival, overhears Pi’s trap and Troy orders him to crush it. In a bid for control, Ronny deceives the reef by claiming he needs a clamp for a talent show, drawing the reef’s attention away from the trap and toward a misguided spectacle. The reef community rushes toward a supposed breakthrough, and Pi confronts Ronny about the deception. Cordelia defends Ronny, arguing that he believes in them and that Pi’s skepticism is what holds the reef back.
Later, Troy arranges a sunset showdown outside the reef and has Ronny lead Cordelia away. As Ronny succeeds, Troy exposes Ronny’s disguise to Cordelia, leaving her horrified. Ronny, who has grown to love the reef and its residents, tries to explain, but Troy sees him as a liability and chooses to discard him. Pi, meanwhile, lies wounded in Nerissa’s shipwreck as Bart and Eddie capture Ronny when his disguise is revealed, bringing him to Pi. Ronny’s remorse is evident as he admits Troy’s coercion and vows to help retrieve Cordelia. Pi, Nerissa, Bart, and Eddie decide to give him a second chance, and Pi frees Cordelia while Ronny blocks the Hench-sharks’ path, urging them to stand up to Troy. Troy overhears the defiance and, filled with fury, swallows the little dwarf shark in a single gulp.
As the reef steels itself for the final battle, Nerissa descends into a mysterious volcano to summon the Sea Dragon, and the volcanic power envelops the turtle in a shivering, ancient glow. The drama escalates as Troy and his sharks attack the reef with ferocity, but the fish outsmart them at every turn. Pi is briefly stabbed by Troy, and Nerissa arrives with the dragon and blasts the invading sharks. Yet Troy sneaks up from behind the turtle and knocks the dragon away, dissolving its form and turning the tides of power in his favor. Pi responds with a Water Ball, drawing Troy’s attention away from Nerissa and toward his ultimate foe.
The climax sees Pi, Cordelia, and Junior unleash the shark trap, using it to hurl Troy out of the sea and back toward the ship. As the humans close in, they administer another vaccine to Troy, who screams in pain and falters on the edge of defeat. The reef erupts in celebration at the fall of Troy, while Nerissa congratulates Pi for becoming a true leader. He acknowledges the danger and sacrifice of those who risked everything, and Nerissa adds with quiet wisdom, “It was my part to play.” The battle ends with Ronny returning to the sea, redeemed by his courage and loyalty, and joins the party among the reef’s survivors as the underwater community revels in their hard-won peace.
