North Pacific Ocean, a group of researchers arrives to study the Great Pacific Garbage Patch and how it affects marine life. While they're filming a video, they're interrupted by the signal of a shark that they've been following for a year. She's coming closer very quickly, so four divers suit up and get in the water.
After looking around for a while, they discover the carcass of a baby whale trapped in the nets. This is the second one they've found in just 3 weeks. When they take a closer look, they notice that its stomach is filled with plastic and it has a bite mark. Suddenly, a shark appears and swims past Chris, but it doesn't hurt him.
Chris realizes this isn't the shark they've been tracking, which means more sharks are coming. His theory is correct. Soon, they're surrounded by sharks that appear to be nervous. Chris's wife, Sophia, is watching everything on camera and notices that all these sharks are female and very big.
After a few seconds, the shark they've been tracking finally shows up. They call her Lilith, and the team is shocked to see that she's 7 m now, when 3 months ago, she was only 2.5 m. Chris thinks this is too dangerous and says they should leave, but Sophia asks him to get a blood sample. A scared Chris tries to pierce Lilith's skin, causing her to swim away in anger.
The divers decide it's time to leave. However, Lilith starts swimming around them very fast. Sophia loses the camera and can only hear how her husband and his team are being killed by the enraged beast. A desperate Sophia then grabs a harpoon and jumps into the water, only to find Chris's arm floating in front of her.
She continues her search and soon Lilith appears in front of her. At first, the shark swims calmly, but suddenly she charges at Sophia, who shoots her harpoon. Lilith gets angry and swims away, but Sophia gets entangled in the fishing net that's stuck on Lilith's fin and is dragged down deeper into the ocean. As the pressure damages her ears, Sophia takes out a knife to cut the net.
Now she can swim back to the surface. 3 years later in Paris, two guys are magnet fishing in the river when they suddenly find a bomb. The cops soon arrive and so does the press, but thankfully the bomb shell isn't working anymore. Officer Otle tells the reporters that finding old shells from World War II in the river is very common, but they haven't caused any trouble so far.
Meanwhile, Sophia is working at an aquarium. She tries to tell a group of kids how important it is to save marine life, but they only tease her for losing her whole team in a shark attack. Drowning in guilt, Sophia leaves the aquarium very distressed. At that moment, she's approached by Mika, who mentions Lilith's tracker.
Mika then takes Sophia to SOS, her ocean conservation group. Every member is a big fan of Sophia's work and has been using her research to protect the sharks. Sophia is introduced to Mika's girlfriend, Ben, who uses her computer to track Lilith's beacon. A confused Sophia says it's impossible because her servers are secure, but Ben explains that nowadays she can gain access to everything, and Lilith has been in the same river in Paris for the past 3 days.
Sharks aren't supposed to live in fresh water. So, this proves that climate change and pollution can change their behavior. Ma thinks that Lilith is just lost and trying to survive. So, tonight she'll dive to try to communicate with her, hoping to help her reach the sea again.
There's also been an accident at the bridge during which a car fell into the water and the driver's missing. Mika wants Sophia to come along, but Sophia thinks it's a ridiculous idea and leaves. At home, Sophia can't stop thinking about Lilith, so she takes out her old investigation equipment and cries as she watches videos of Chris and her team.
When the clip shows Lilith, Sophia has a breakdown and finally opens the beacon tracker, which confirms that Mika's story is true. That night, Mika and Ben sail on the river until they reach the bridge. Mika jumps into the water and finds it hard to see because of the pollution. When she comes across the car, she notices multiple bite marks.
On the surface, Ben hears the cops approaching and pulls the rope to warn Ma. However, Mika wants to keep investigating, so she disconnects the rope. At that moment, Mika sees something huge swimming near the car, but when she searches the area with her flashlight, she sees nothing. Hearing the police sirens above, she decides to resurface.
Ben is already running away, so only Mika gets arrested. On the police boat, Mika tells Adel that she had been cleaning the river, but he doesn't believe her. On the way to the station, the cops stop to remind some vagrants that the city wants this area clear for the triathlon. By the time Sophia gets to the bridge, everyone is gone, but her tracker still detects Lilith in the area.
She follows the beacon and finds the vagrants camp, but it's empty except for their dog. The little guy keeps looking at the river and growling. In the darkness, they don't see the shark Finn under the bridge. The next morning, Otle keeps interrogating Mika until she tells him about the shark. Otle doesn't believe her, but he's interrupted when another cop mentions an emergency.
Near the river, Sophia is still tracking Lilith and notices that she's close, so she warns a kid to get away from the water. At that moment, she gets a call from the police and is taken to see Otel and Mika, who are also by the river. It turns out they found the body of a vagrant maul to death. Seeing the injuries, Sophia confirms that a shark did it.
They take this information to Chief Onel who doesn't believe a shark could be in the river and wants proof first. Otle and his team decide to go diving for proof and Sophia goes with them to guide them with her tracker. Three officers jump into the water and find a huge dead fish with big bite marks. Mika has been following the group on land and thinks they're going to kill the shark.
So, she calls Ben and asks her to hack the beacon to disable it. Ben is successful and soon Sophia and the cops lose the signal. Underwater, the officers see something swimming in the dark around them. Otle notices the rope tensing, so he and Sophia immediately start pulling to help the crew get back on the boat. The divers explain that they saw something big down there and make check the sonar, which confirms Lilith's in the river.
Now thinks that Sophia turned off the beacon and blames her for putting the team in danger. When her nose starts to bleed, Sophia leaves the boat and sees Mika nearby. She follows her to the office and they argue about what happened. Mika insists that sharks don't attack for no reason and that what happened to Chris was Sophia's fault.
This upsets Sophia and causes her to leave. At the police station, Adel tells Anel that they must block the river area, which would require cancing the Olympic triathlon. Anel worries about the mayor's reaction. Adel searches Sophia online and finds her speech on climate change. He also discovers what happened to Chris and her team.
This makes him think of all the friends he lost during the war when he was in the army. In the meantime, Mika streams on social media to tell the world about the shark in the river. She explains that Lilith's behavior was affected by climate change and she doesn't deserve to die, so they should work together to save her. After seeing the video, Sophia tries checking the beacon, but it's still not working.
On her way out of work, Otle approaches Sophia and apologizes for starting on the wrong foot. He also asks her to come to the meeting with the mayor. At the mayor's office, on Jel, Ot and Sophia try to explain how dangerous the situation is. However, the mayor has already spent a lot of money and refuses to look bad in front of the press, so she refuses to cancel anything and orders the cops to deal with the shark before the event starts.
Afterwards, Sophia tells the officers that their best chance is to lure Lilith to the bridges and sedate her. Then they can capture her and the oceanographic institute can take over. Sophia shows them a video explaining that once they sedate Lilith, they only have a few minutes to get her ready, otherwise she'll esphyxiate.
At that moment, Ben arrives and tells them that Lilith is in the catacombs near the sewage reservoirs. She also reveals she came here without telling Mika, who believes that Lilith can be guided using a sonar. Ben reactivates the tracker in Sophia's computer and shows them an old access tunnel under the canal that connects the river to the catacombs.
Sophia and the cops use the boat to reach the old access tunnel, but they must continue on foot because it's very narrow. The activists are already there, and Ben tries to convince Mika not to dive to no avail. Mika jumps into the water with the sonar right before the cops arrive to kick everyone out. However, Mika refuses to give up and tells everyone to stay.
At that moment, a large fin appears in the water and starts circling Mika. To everyone's shock, a second fin soon joins them. Judging by the size, it seems Lilith has given birth. The baby shark approaches Mika and she pets it like a dog, ignoring Sophia's warnings. Suddenly, the fins disappear and Sophia believes they're about to attack.
Yet, Mika still refuses to move. Adel jumps into the water and tries to drag Mika out, but she bites his arm and swims away. Lilith then jumps out of the water and eats Mika with just one bite. The shark size causes a huge wave that makes several people fall into the water. To Sophia's surprise, Lilith bites her baby to kill it, probably because of the human smell.
With the help of another officer, Otle gets out of the water and the cops guide the crowd out of the tunnel. However, people keep slipping and falling, so Lilith kills every person she can reach. Ben is accidentally pushed into the water and Otto tries to get her out, but another person pushes her down while trying to escape, and Ben ends up getting killed, too.
Many of the activists they do manage to rescue her missing limbs, and a furious opens fire, but all his shots miss. When an officer leans in to help more people, the panicking victims accidentally drag him into the water. His partners try to get him out, but Lilith is stronger and soon drags him back to eat him. In the tunnel, the running crowd piles up and many people end up drowning as other victims walk over them.
Moments later, doctors and nurses are overwhelmed with work as all the victims are taken to the hospital. Sophia blames herself for what happened, but Otto assures her she did what she could. The baby shark's body is retrieved from the water, and Sophia gets to study it in a lab. She notices all the mutations that confirm it's adapted to live in fresh water.
Sophia also confirms the baby is female, and when she cuts her stomach open, she finds several more babies. This doesn't make sense since this shark is only a little over one month old. She concludes the mutations are allowing Lilith and her offspring to reproduce through parthonogenesis, which is a process that allows a baby to grow from an egg without fertilization.
This means Lilith is the first of a new species, and if they don't eliminate her, she'll keep making more like her. The next time the team meets with the mayor, she complains about the 12 deaths. A man from the army says they should have reported this and they explain they did warn the mayor, but she claims that they didn't tell her enough.
Sophia wants her to tell the truth to the press and cancel the event. However, the mayor kicks her out and tells her team to hush this up. In the afternoon, when the mayor appears on TV, she says the stories about sharks and the river are lies and the triathlon will still be held. In a bar, Sophia and are frustrated about the situation.
Sophia asks why he left the army. So, he explains that during a rescue mission, he panicked and left his teammates to die. He doesn't want to be a coward again, so Sophia shares an idea. It'll be off the books, but some officers agree to help anyway. With the help of two bomb experts, they'll look for the nest in the crypt and blow it up.
They'll have to move fast because they'll only have 3 minutes to swim away before the bomb brings the crypt down. Sometime later, the area near the river is crowded with tourists for the event, and several soldiers are sent to keep an eye on the water. Two officers are supposed to offer support from the boat, but the soldiers don't let them sail near the crypt because the river has been blocked off.
Since Anel is part of the mayor's security team, she tells the soldiers to let her people pass by pretending they're part of the operation, too. Meanwhile, Otle and Sophia dive in and look for the nest. When they light up a flare, they discover dozens of baby sharks above them. One of the sharks swims toward them, but they move out of the way to avoid it.
On the surface, the mayor starts the event and all the competitors jump into the water for the swimming competition. While the bomb experts install the explosives, Sophia sets up a sonar to attract Lilith, but she doesn't show up. Suddenly, the baby sharks attack one of the bomb experts, killing him in seconds and causing him to drop the detonator.
Sophia tells the others to stick together and not move, but the other expert decides to swim ahead to retrieve the detonator. Right after he presses the button, the sharks surround him and eat him, too. With only 3 minutes to escape, Sophia grabs Otle and activates a propeller to swim away. The bomb explodes behind them, killing all the baby sharks.
The cops on the boat see the burst of water and sail toward it. They're relieved to see Sophia and Otle come back alive, although Otel is wounded. While the officers take care of Otel, Sophia dives in again and is startled by Lilith, who wasn't reached by the explosion. Sophia immediately returns to the surface and tries to warn the others, only for Lilith to hit the boat and send it flying.
All the cops fall into the water and Lilith proceeds to eat them one by one except for Otle, who reunites with Sophia, and together they climb on top of the boat. Afterward, Lilith approaches the swimmers and starts killing them as well. turning the river red in the process. The crowd doesn't notice until Lilith jumps out with a swimmer in her mouth.
The competitors swim away as fast as possible while the military boats open fire, but Lilith's mutation has made her so fast that she can dodge them all. On Jel and Otle try to tell them to stop because the river is full of old bombshells. However, they aren't heard. While the crowd panics and runs away, another cop appears in a new boat to finally rescue Ot and Sophia.
The soldiers run to the bridge to continue shooting, and as predicted, they end up hitting a bombshell. This one explodes and only causes a small burst of water, but this creates a chain that makes the other bombs go off, too. The resulting explosion is so big that it brings down two bridges, and even the manholes on the street lose their lids.
As the crowd keeps running, they accidentally push more people into the water. This explosion also creates a huge tsunami that rises over the river and quickly floods the city. Several people, including the mayor, fall into the river and aren't seen again. Sophia swims quickly to rescue Otle and pulls him to the top of a sunk building.
When they look around, they discover they're surrounded by sharks. Thanks to the river connecting to several other water sources, Lilith and her offspring spread all over the world and begin terrorizing all major cities.
