In the ocean’s depths, Pup, Diong Chae Lian, a curious young brownbanded bamboo shark, and his bigger friend Julius, Gavin Yap, roam the seabed until trouble arrives: two human divers steal several of Pup’s unhatched siblings from the reef, despite Pup’s earnest attempts to stop them. To lift Pup’s spirits, Julius invites him to a late-night dinner aboard his sunken ship, but the young shark is too upset to eat, a sign that something deeper is stirring beneath the waves.
Pup’s world brightens a little when he meets Octo, Kennie Dowle, an inventive octopus who loves tinkering. Octo proudly shows off an underwater car of his own making and offers Pup a test drive that fills the youngster with wonder and a growing sense of possibility. This moment of invention sits against a grimmer current: a factory on the surface spews pollution, and Murray, Mike Swift, a hostile moray eel, rallies an army of spider crabs to press the ocean bed under siege.
Spin, the intelligent stingray Jay Sheldon, discovers a discarded restaurant menu and carries it to Julius’s three pilot fish butlers. The menu—a surprising collection of chicken meals—only exists on land, hinting at the looming clash between sea and shore worlds. Spin and Myrtle, the sea turtle Christina Orow, guide Julius toward a beach, but he is nearly stranded as he tries to snatch a meal from Heather, a hen Adila Shakir. He manages to return to the safety of the sea, but the encounter leaves a mark.
On a subsequent hunt, Julius is snagged by a heavy fish hook and hauled onto a fishing boat. He is almost finned alive by a group of fishermen, but a massive wave bursts onto the deck, scattering the danger and buying him a perilous moment of escape. Pup’s next grim discovery comes on a wharf: his unhatched siblings are being moved into a fishmonger’s shed. Myrtle and Octo refuse to join Pup in this risky mission, and Pup overhears them talking about his unusual ability to breathe on land, which only deepens his resolve to act.
Myrtle and Julius, having learned more from Spin during a tense interrogation, realize Pup’s danger and coax Octo into joining the rescue. Pup finds the shed where the eggs are kept, but must hide when the fishmonger returns. The stakes rise as Octo builds a plan using submarine wreckage to create a robotic suit that will let Julius and his pilot fish venture onto land, and a rocket is pressed into service to launch the suit toward the shore. Yet Octo’s nerves bite back at him: he worries the contraption might fail, and his misgivings add tension to the mission.
The suit crashes into a chicken farm and the group is captured by roosters who haul them to the fishmonger’s shed. Back underground, the shark eggs begin to hatch, and Pup seizes the chance to whisk them away from the shed, aided by Myrtle. Hearing that Julius is in trouble, Pup sets off to rescue his friend, aided by a coconut crab and the roosters who reluctantly lend a paw.
With the eggs and two hatchlings safe, Myrtle and Octo rally to support, and Spin reappears, having previously aided them. Octo also pulls sea mines from an old stash to block the factory’s polluted pipes, a cunning move that helps calm the mounting crisis. The team frees Julius and his pilot fish, and a brief but tense fight with the fishmonger and his divers ends as the shed catches fire and the humans give chase on dune buggies and motorcycles. The roosters use their quick wits to separate the divers from danger.
On the beach, Pup is briefly captured by the fishmonger, but Julius rescues him just in time. The fishmonger careens off the pier, the buggy collides with sea mines, and the factory explodes in a dramatic blow to the pollution that started it all. The fishmonger and divers are arrested, and the deep-sea invasion loses its momentum as Julius climbs out of the land suit, returns to the reef, and drives away the remaining intruders. With danger receding, Pup and friends raise the newly hatched sharks, while the roosters entertain themselves with Octo’s suit, which eventually breaks and sinks back into the ocean, leaving a hopeful note for the reef’s future.
