Plot
In Australia, hungover lifeguard Josh is woken up by his friend and fellow lifeguard Rory, who asks Josh if he regrets proposing to Rory's sister Tina. Josh visits Tina, who discusses their upcoming move to Singapore. As Rory enters the ocean to set a buoy for Josh, a great white shark kills a man in the water. Alerted to the danger, Josh quickly takes a jet ski to save Rory, who is killed by the shark before Josh reaches him.
A year later, Josh works for a supermarket in Coolangatta. While stocking shelves with his co-worker Naomi, he sees Tina and her new boyfriend Steven returning from Singapore. At the same time, teenagers Kyle and Heather are making out in their car. A young woman named Jaime is caught shoplifting and temporarily evades the security guard by meeting up with her boyfriend Ryan, who also works at the store. Store manager Jessup catches up with her, fires Ryan, and calls the police. Arresting officer Todd arrives and is revealed to be Jaime's father. Jessup is suddenly held at gunpoint by a robber named Doyle. Tensions escalate and Doyle's masked partner appears, shooting assistant manager Julie.
At the height of the commotion, a tsunami wave descends on the city and floods the supermarket. Doyle's partner is killed by the flood, and the survivors are forced to take shelter on top of shelving units. As they try to find a way out, security guard Bob is dragged underwater and killed. It becomes apparent that a 12 ft (3.7 m) great white shark has been washed into the store by the tsunami, and a broken wire threatens to electrocute them all. Steven volunteers to shut off the power and the others dress him in crude armor made of shopping carts and shelves to protect him from the shark. He succeeds, but loses his oxygen tube and drowns. Despite their previous conflicts, the survivors work together to get Jessup into a ventilation shaft to find help. Crabs slide out of the vent, startling Jessup and causing him to fall back. He manages to grab hold onto the vent, but the shark jumps out of the water and bites him in half.
Kyle, Heather, and Ryan have been cut off from the others and are trapped in the parking garage, surviving the flooding as their cars were sealed when the water hit. Ryan helps the couple escape from their flooded car, but a second great white shark is revealed to have entered the parking garage. Kyle abandons Heather's Pomeranian dog Bully, and they manage to get to temporary safety. After several unsuccessful attempts at luring the shark away, Ryan decides to join them on top of his flipped van, but the shark chases him. Ryan successfully climbs onto the van, but Kyle falls and is eaten.
Inside the supermarket, the remaining group makes a plan to catch the first shark so they can swim to safety. Jaime manages to swim to the butcher section and grab a hook with meat to use as bait. The shark does not go for the bait, so fellow survivor Kirby grabs a hook and puts it through Naomi's shirt, using her as bait. Kirby is revealed to be Doyle's partner, who changed out of his clothes and mask so the others wouldn't know his identity. Doyle stabs Kirby with a makeshift harpoon and throws him into the water with the hook. Naomi is pulled from the water as the shark devours Kirby, catching its jaw on the hook and ensnaring itself in the trap. Josh apologizes to Tina, feeling guilty over Rory's death, but Tina reassures him and kisses him.
Bully is found alive in the parking garage, and Heather's newfound hope inspires Ryan to start banging on the pipes, calling for help. Jaime hears Ryan's call and goes to rescue him with Josh. Below, they are alerted to the second shark's presence. Josh and Jaime find her dad's car, which has a shotgun and taser inside. Josh kills the shark with the shotgun, and the four of them get back into the supermarket. A tremor strikes as they are all swimming to the entrance, breaking the first shark loose. Josh kills it with the taser as Doyle finds a truck jammed in the entrance and rigs it to explode, breaking a hole in the debris and freeing them. The survivors leave the supermarket, reaching the severely damaged city, and Tina asks Josh what to do next.
Out at sea, a seagull swoops low over the water, and a shark jumps out and devours it.
