The film starts at a swim relay at the University of Florida. Haley Keller (Kaya Scodelario) competes, but she just misses the best time. It brings back a memory of her losing a relay as a child and feeling bad, but her father Dave (Barry Pepper) would continue to encourage her to keep up her hard work.After the relay, she receives a video call from her sister Beth (Moryfydd Clark). She informs Haley about the impending category 5 Hurricane Wendy that is about to strike Florida. Beth expresses concern since she hasn't heard from their father, and she thinks he is still around the area. They have a slight argument since it appears that Haley took Dave's side in her parents' divorce, while Beth took their mother's side. Beth just asks that Haley get in touch with Dave.Haley starts to drive out of town as per evacuation notices, trying and failing to get in touch with Dave. She runs into an old friend/ex-boyfriend of Beth's named Wayne (Ross Anderson), who is attempting to direct traffic as the storm starts to hit. He shows her where to go, but, out of concern for her father, Haley turns around and drives down a path to get back to find Dave.Haley goes to Dave's condo to only find his dog Sugar there. The rest of the place is empty. Haley calls Beth to inform her, and Haley suggests that Dave is at their old family home. Haley heads there with Sugar against Beth's wishes, especially as the streets have started to fill up with water. When they get to the house, Haley doesn't find anyone upstairs. She calls his phone and finds it in the kitchen. His car is still in the garage. Alerted by Sugar's barking, Haley goes into the basement where she finds Dave unconscious and with a badly broken leg. She tries to carry him upstairs, but they are attacked by an alligator that has made its way down there. Haley pulls herself and Dave to safety before barricading themselves behind pipes.Haley tries to help tend to Dave's wounds, but he tells her she should get out of there on her own. She refuses to leave without him. Dave explains that his leg got broken after escaping the gator. Haley wants to retrieve her phone that she dropped in the struggle, so she attempts to get through by staying hidden. She reaches it, but a second gator appears behind her. While trying to escape both gators, one of them crushes the phone. Haley's leg gets bitten, but she manages to blind one of the gators in one eye, while Dave beats the other gator's head in. She gets away from both of them and stays in a corner to treat her wound. Dave also painfully manages to fix his leg.With the water levels in the basement quickly rising, Haley tries to find a way to get help. She flashes a light to two people, Stan (Ansom Boom) and Lee (Ami Metcalf), who are across the street at the gas station trying to loot it with their friend Marv (George Somner). Stan starts walking over, but the gators get Lee in the boat before getting to Stan. He tries to yell out to Marv to warn him, but Marv watches as Stan is messily torn up outside the store. Marv sees too late in the mirror that a gator has come for him, and it immediately makes a meal out of him.Dave suggests that the only other way out is through a passage leading up back to the living room, as taking the stairs will lead to the gators. Haley tries to push the hatch open, but it is blocked by a drawer weighing it down. Outside, Haley and Dave notice Wayne and his police partner Pete (Jose Palma) coming by to check for people who may have been left behind. Pete stays on the boat while Wayne goes inside. Pete notices that the boat's turbine is caught in seaweed. Wayne enters the house to try and find Haley, but he is grabbed by a gator's jaws. Pete is caught outside by another gator as well, and he is brutally torn to shreds by other gators joining in. Haley makes it to Dave's side and begins to cry, thinking they will die right there. She also has been blaming herself for her parents' divorce, thinking that she made Dave turn his priorities toward her and swimming. He assures her that none of it is her fault and that she and Beth kept them together until they both moved out. Dave encourages Haley to keep pushing forward.Haley goes for Wayne's gun, but a gator bites into her arm. She shoots repeatedly until it is finally dead, and the bullets have run out. Haley then figures the only other way out is through the storm drain, which is where the gators came in from. She swims through it and avoids being spotted by the gators, but she does see that they have already laid eggs, and some of them are hatching. She gets out safely and runs into the house to pry the hatch door open with a crowbar, just as the basement has completely filled with water. Haley pulls Dave out, but he is unconscious. She manages to revive him, and they make their way outside.Haley and Dave grab Sugar and attempt to make their way to the looters' boat across the street, but they want to do so without making splashes so that the gators won't hear them. They are successful in getting the boat, but the flood waters become too much, and it pushes the boat back into the house. Haley finds a radio and tries to call for help. Dave tries to protect Sugar from a gator, and he gets his arm bitten off. Haley is trapped in the upstairs bathroom with a gator as the water level rises in there, but she manages to trap it in the shower before getting back to Dave.The two take Sugar outside as they head to higher ground. Another gator attempts to attack Haley, but it gets swept away by the current. She and Dave lay on the roof as a helicopter comes near. Using a road flare she found, Haley uses it to signal it toward them. It works, and the people in the helicopter lower something to pull them up to safety.