Medical student Nancy (Blake Lively) has traveled to Mexico to surf the same waters her late mother visited while she was pregnant with her. She catches a ride out to the beach from an agreeable local and finds herself out on her own when her friend stays behind after a night of partying too hard. She hits the waves and finds solace and satisfaction in the water, but she makes a potentially fatal mistake when she wades back out for one more run before heading back to the hotel. She finds a dead whale a little ways out and soon, its killer: a large, hungry shark bent on feeding on everything in the water. She's bitten in the leg, tearing open a large gash of her flesh. She manages to reach a rocky formation and find temporary safety, but the shark isn't about to let its meal slip away after a respite. It circles her rock, leaving her wounded, tired, and with no avenue of escape....