NOTE: This spoiler was submitted by Jeremy
Medical student Nancy Adams (Blake Lively) is headed toward a beach for a personal vacation. The beach was a favorite spot of her mother, who recently passed away from cancer. Nancy is driven there by a man named Carlos (Oscar Jaenada). She is supposed to be there with her friend Anna, but she claims to have gotten sick, leaving Nancy to take the trip alone. She offers him some cash for his services, but he declines.
Nancy gets into her surf gear and hits the waves on her board. She comes across two other surfers who know she is a visitor. Nancy joins the two of them as the waves get nicer.
Later, Nancy has a video chat with her sister Chloe (Sedona Legge) to tell her all about the beach and how their mother was right about how it's like paradise. Nancy then talks to her father (Brett Cullen), who knows she is taking the trip for her mother. He expresses concern to Nancy over her grief and actions.
Nancy swims out past the shallows of the ocean to catch some more waves. As she gets further from the shore, she sees a whale carcass floating up top. Sensing trouble, Nancy tries to head back to the shore on a wave, but as she rides it, a great white shark slams into her and knocks her off the board. Nancy climbs back on and tries to paddle back, but the shark bites deep into her left leg, spilling a lot of blood into the water. Nancy struggles to escape and swims to a rock where the shark can't reach her. She takes the ankle strap from her board and uses it as a tourniquet to stop the bleeding. Nancy sees the other two surfers and tries calling to them for help, but they don't appear to hear or see her. She wraps her suit around her leg.
It starts getting darker as Nancy tries to get an opening away from the shark. Her only companion is a seagull with an injured wing. Nancy tries to eat a tiny crawling creature, but she quickly barfs it up.
On the shore, she sees a drunk man passed out in the sand. She calls out to him and manages to get his attention. Nancy points toward her bag for her to call for help. The man just steals her phone and wallet. He sees her board floating in the shallows and he walks into the water to get it. Nancy yells to him to get away from the shark. She then sees the shark attack the man as he heads back to the shore. The shark tears the man in half, and the top half tries crawling away until he dies.
Nancy gets colder as the night falls. She puts her suit back on to keep warm. She also loosens the tourniquet to allow blood flow in the leg. Nancy then tries to stitch herself up using her earrings and necklace chain, but it's too painful for her.
In the morning, the other two surfers return to the ocean. Nancy calls to them as they ride their boards into the water. She warns them about the shark, but one surfer assures her there isn't one. He's proven wrong when the shark jumps up and takes eats him. The other surfer rushes toward the rock, but the shark pulls him down. He resurfaces quickly by the rock where Nancy tries to pull him up. The shark pulls him down, leaving only his helmet with a camera on top, which captures him getting eaten.
Nancy observes the shark's movements as it circles the rock, estimating that it swims about 32 seconds around another rock, which leaves her with time to swim and reach the helmet. She takes that chance and makes it to the little rock, but she misses the helmet before the shark returns. As she goes back to the bigger rock, she sees the helmet again. Nancy reaches for it as the shark lunges toward her. The shark hurts itself as it scrapes against the coral. Nancy manages to get the helmet. She makes a video for anyone that finds the helmet. She shows how bad her leg has gotten, with gangrene setting in, and she leaves a message for Chloe and their father. Nancy then tosses the helmet toward the shore.
Nancy pops the seagull's wing back into place and then lets it float away on a broken piece of surfboard.
Nancy sees a buoy several feet away. She makes her plan to reach it and try to bring attention to herself. As she prepares herself, the shark swims toward her. A school of jellyfish rises up. Nancy thinks the shark can't harm her if it runs into something that can hurt it. Nancy jumps into the water and swims toward the buoy, and the shark follows closely. She gets stung on her shoulder, while the shark eats some jellyfish and gets stung too. Nancy reaches the buoy and finds a flare gun. She grabs the shots and fires a few in the air as she spots a boat in the distance. Unfortunately, the boat is already turned away, and it never sees her.
The shark starts to circle the buoy. Nancy fires a flare directly at the shark. Now pissed off, the shark starts slamming into the buoy, breaking the chains that keep it up. It manages to loosen the buoy and starts chomping into it. Nancy struggles to keep herself at a distance and to hold onto the buoy. While underwater, she notices the chain holding the buoy to a spiked base. At the right moment, Nancy gets the chain on a hook stuck in the shark's side. This pulls her down toward the base as the shark chases her. She pulls out at the right moment, causing the shark to swim directly onto the base and get impaled.
On the shore, a little boy finds the helmet camera and sees Nancy's video. He runs to get his father, who is Carlos. The two find Nancy washed up on the shore, dehydrated and weak. They run to help her. Nancy sees the seagull standing close to her. Before she passes out, Nancy sees a vision of her mother smiling down on her. Nancy simply says, "I'm okay."
One year later, Nancy is doing better, albeit with a big scar on her leg. She returns to the beach with Chloe and their father. Nancy brings Chloe into the water to teach her how to surf.