Aspiring swimmer Haley Keller (Kaya Scodelario) receives a call from her sister Beth, who informs her that Category 5 Hurricane Wendy is on its way to Florida, and advises her to get out of the state. Concerned for the safety of her estranged father, Dave (Barry Pepper), Haley goes to check on him at their old family home in Coral Lake, against the instructions of Florida State Police.
At the house, with the help of the family dog Sugar, Haley descends into the crawl space underneath the house eventually finds her father unconscious and badly wounded. When she tries to drag him out, her exit is cut off by large alligators that Dave believes got into the house via a storm drain open to the crawl space. The alligators are too large to fit under the pipes in the crawl space, allowing Haley and Dave to find safety at the back of the basement.
As the hurricane intensifies and the house begins to flood, Haley attempts to navigate around the alligators before she and her father drown. She tries to get the attention of three people in the gas station opposite the house, but they are killed by three more alligators. Two police officers also arrive later on but they are both also savaged by the alligators. In a last-ditch effort to escape, Haley swims to the storm drain where she discovers that the alligators have made their nest and laid eggs.
Haley successfully kills one alligator using a gun retrieved from the body of one of the police officers, shooting down the alligator’s throat as it nearly bites her hand off. She then swims through the storm drain and out into the flooded street, and makes it into the house to crow-bar the living room floor open, saving Dave from drowning in the basement. Outside, Haley, Dave and Sugar carefully make their way onto a boat, just as the eye of the hurricane moves over the neighborhood. The floodwaters break the nearby levees, flooding Coral Lake even more and crashing them back into the house, where they get separated.
While Dave and Sugar make their way up the stairs, Haley navigates around the kitchen and uses a discarded police radio to broadcast a distress signal to authorities. Dave is attacked by one of the alligators and loses an arm. Meanwhile Haley manages to trap one alligator in the shower. She then attempts to flag down a rescue helicopter from an upstairs bedroom but is attacked by another alligator, which attempts to drown her in a death roll. While Dave and Sugar escape to the attic, Haley stabs the alligator in the eye with a flare. She joins her dad and Sugar on the roof, narrowly avoiding being mauled by a fourth alligator before it gets swept away. Haley then lights a flare and flags down the rescue helicopter as Dave watches proudly.