To save Paris from a bloodbath, a grieving scientist is forced to face her tragic past when a giant shark appears in the Seine.
Summer 2024. Paris is hosting the World Triathlon Championships on the Seine for the first time. Sophia, a brilliant scientist, learns from Mika, a young environmental activist, that a large shark is swimming deep in the river. To avoid a bloodbath at the heart of the city, they have no choice but to join forces with Adil, the Seine river police commander.
The scientist Sophia Assalas is researching the shark Lilith, a.k.a. Beacon Seven, in the Great Pacific garbage patch with her partner Jade on board and her team of four-men divers, including her husband Chris. When they see Lilith, Chris notes that the shark has grown more than the usual and he tries to take a blood sample for the research. However, Lilith attacks the divers, slaughters them and flees from Sophia. Three years later, the traumatized Sophia is working at an aquarium when the twenty-year-old activist Mika, who works with a group of environmentalists, brings Sophia to her base and her colleague Ben shows the beacon of Lilith trapped in the Seine. Mika searches for Lilith but is arrested by the River Police under the command of Sgt. Adil. She calls Sophia but the police officers are reluctant to believe that there is a shark in the Seine. But when a dead man is found with shark bites, Adil believes in Sophia and team-up with her. They ask the Mayor of Paris to call off the triathlon that will happen in the Seine in a couple of days, but the politician refuses and a tragedy happens in Paris.—Claudio Carvalho, Rio de Janeiro, Brazil
Near the Great Pacific Garbage Patch, marine researcher Sophia Assalas and her team search for Lilith, a tagged short fin Mao shark. Her husband Chris leads a dive team of Sam, Juan and Tom, leaving Sophia and Jade inboard. The team first observe an unusual sight of makes hunting in a pack, before they locate Lilith, who has grown much larger since she was last sighted. With the shark appearing non-aggressive, Chris attempts to take a blood sample from Lilith, but the shark suddenly snaps and kills him, Sam, Juan and Tom. Leaving the safety of the boat, Sophia goes after Lilith, but becomes entangled in a net and dragged far beneath the surface as the shark escapes into the depths. She manages to free herself, but not before suffering injuries due to the severe change in pressure.
Three years later, Sophia works at an aquarium in Paris. Environmentalists Mika and Ben inform Sophia that Lilith's tracking beacon is still active and that she has traveled from the Pacific and up the River Seine and is seemingly trapped in the city. After a man is found dead with wounds from shark bites, police diver Adil recruits Sophia to help find and kill the shark, but Mika and Ben turn off the beacon before they can do so in the hope that they can rescue her later.
Sophia and Adil petition the Mayor of Paris to postpone the upcoming triathlon. The mayor refuses, emphasizing the event's importance for focusing global attention on Paris during the run-up to the Olympic Games which billions of euros have already been spent on. Mika reveals Lilith's existence to the public, and leads a group of supporters into the city's catacombs, where the city's waste water reservoirs are, to find her. Ben tells Sophia of Mika's plan, and she and Adil's team head down into the catacombs to find them after reactivating Lilith's beacon.
Mika activates a pulse to lure Lilith to where her group have convened, and swims out to the middle of the reservoir. Adil's team arrives just as Lilith appears with a juvenile shark, Lilith's offspring. Mika does not heed to their warnings to get out of the water, and after she pets the juvenile shark, Lilith attacks and kills her, causing a mass panic that leads to many injuries and twelve deaths, including Ben and Leopold, a police officer on Adil's team. In the aftermath Sophia and Adil find the juvenile shark dead, and after examining it they discover that it's a female that mutated to adapt to fresh water and became pregnant through parthenogenesis, like Lilith. Sophia concludes that if Lilith is not caught, she may give birth to more mutated offspring. The Mayor, deflecting blame from herself, orders them to kill the shark, and still refuses to cancel the triathlon to be held in the river the following day. She takes part in a TV interview downplaying the danger the sharks present.
Sophia and Adil devise a plan to lure Lilith out of the catacombs and blow her up with the help of explosives experts Poiccard and Berruti. As the mayor declares the triathlon open, and swimmers take to the water, Adil's team Caro, Adama, Angèle and Markus begin to execute the plan. Underwater, Sophia and Adil encounter a large school of juvenile sharks as they set up their trap. The sharks kill Poiccard, Berruti and Adama before Adil sets the explosives off, seemingly destroying the school of young sharks and leaving the gigantic Lilith as the only survivor. She capsizes the police boat, killing Caro and Markus, and she heads towards the triathlon. The mayor and the spectators look on in horror as several swimmers are attacked and killed. The military open fire on Lilith, despite warnings from Adil that there are live shells on the riverbed. The disturbance in the water from their gunfire and Lilith's movements causes unexplored ordnance at the bottom of the river to shift and detonate, throwing the Mayor, Angèle, and many spectators into the water. The resulting chain reaction of explosions destroys several bridges and creates a tsunami that incredibly entirely floods the center of Paris. Sophia and Adil are left stranded on the roof of a building in the middle of the flood, surrounded by Lilith and the juvenile sharks that have survived the explosions.
As the end credits suggest, Lilith and her offspring have proliferated in the major river cities of the world (Paris, London, New York, Bangkok, Venice and Tokyo), and seem to have, as Sophia feared, colonized the entire world.
