Studying the effects of climate change off the coast of Mozambique, a marine biologist and her team confront three genetically enhanced bull sharks. Now, a new bloodbath is waiting to happen in the name of science. Will humans never learn?
Dr. Emma Collins and her team are spending their third summer on the island of Little Happy studying the effect of climate change on the great white sharks who come to the nearby nursery every year to give birth. Along with the last two inhabitants of this former fishing village, their peaceful life is disrupted when a "scientific" team led by her ex-boyfriend and marine biologist Richard show up looking for three bull sharks who we soon learn aren't just any bull sharks.
Off the coast of Mozambique, somewhere in the middle of the vast Indian Ocean, the rapidly sinking artificial islet known as "Little Happy", and its last two residents are facing the effects of global warming. There, the passionate marine biologist, Dr Emma Collins, and her team are studying the impact of climate change on Great White sharks, who come to mate in the peaceful marine conservatory, when, unexpectedly, Emma's old flame, Richard Lowell, arrives on a research vessel. Now, in high hopes of capturing three rogue, genetically enhanced bull sharks, Richard drags Dr Collins in a fierce battle of survival, as the trio of fast underwater killers have already picked up the fragrant scent of human blood in the water. In the wake of the Akhelios Complex massacre in Deep Blue Sea 2 (2018), a new bloodbath is waiting to happen, of course, in the name of science. Will humans never learn?—Nick Riganas
Dr. Emma Collins carries on her late father's efforts to protect great white sharks on Little Happy, an artificial island off the coast of South Africa. Miya, Spinnaker (also known as "Spinn"), and Eugene Shaw-Emma's father's best friend and brother figure-are on her team. Nandi and Bahari, Emma's brother, are also residents of Little Happy. Bella, a bull shark that caused havoc in previous years, is found dead after becoming entangled in fishing lines. Dr. Richard Lowell and his team found her on their Thasos boat. The three remaining Bella pups are now adults and the only ones left in the litter when they arrive at Little Happy.
It turns out that Emma's ex-boyfriend is Richard. Emma joins Richard and his group as they dive to hunt the bull sharks, and they find three great white sharks that the bull sharks have eaten. Soon after, Brown, one of Richard's team members, is discovered dead after the bull sharks cut him in half. One of the largest bull sharks is tranquilized and taken down with Emma's assistance, but Richard suffers injuries.
Later, when Bahari goes fishing, one of the two bulls that are left attacks and eats him. Later, Nandi finds out that his arm was cut off. Emma confronts Richard after losing a friend. He tells her that these bull sharks are more intelligent than people by reciting the events of the first two movies. He asserts that he can attract her children, who are aware of their mother's death, by using the beacon that was attached to her. He concludes by warning that if these sharks breed, calamity will result. Lucas, the leader of Genotic Labs' mercenaries, overthrows Richard's command and decides to kill everyone while stealing the captured shark. They set a mine to demolish everything, not realizing Emma and her group are using an underwater drone to spy on them.
Emma accuses Richard of plotting their murder and confronts him once more. They and Emma's team are attacked by Lucas, who overheard their conversation and used explosives to seriously damage nearby structures on the island. Emma manages to get away during the fight, but Richard is taken prisoner by Lucas's men, Schill and Earls, and taken to the Thasos.
When Richard decides to support Emma, he leaps from the boat to join her, only to be killed by one of the sharks. While Earls and Shaw are fighting, a bull shark kills Earls. A bull shark kills Spinn in the meantime, and Miya is thought to meet the same end. Schill, who has been ordered by Lucas to kill Emma, attacks her after she deactivates the mine. Sally, Emma's "friend" since arriving on Little Happy, a great white shark that eats Schill, saves her. When Emma plants the mine on the Thasos, Lucas detonates it, causing his boat to capsize and releasing the captured shark.
In order to use explosive darts to kill two of the three bull sharks, Shaw gives his life. After surviving the recent explosion, Lucas attacks Emma, but she and Nandi kill him. Emma then lures the final bull shark into the garbage compactor, where it is killed.
Little Happy is destroyed as Nandi and Emma run away, and Sally says goodbye to Emma before vanishing into the sea. Nandi and Emma discover a wardrobe while navigating, and Miya is discovered alive after hiding inside. The three friends return to civilization after being reunited.
