Fun-loving sisters Lisa (Mandy Moore) and Kate (Claire Holt) are on holiday in Mexico enjoying a slice of paradise as Lisa gets over a recent breakup. When they meet two gorgeous locals who offer to take them cage diving with great white sharks, Kate persuades her reluctant sister that some excitement in her life is exactly what she needs. Far out at sea, under the guidance of Captain Taylor (Matthew Modine), the rickety metal cage is lowered into the water and the two girls fearfully begin their descent. However, when the cage suddenly breaks away from the boat they find themselves plummeting 47 metres to the seabed, trapped in deep darkness. With no way up, less than an hour of oxygen left and a group of hungry predators circling, their adventure rapidly turns into a living nightmare and their fight for survival begins.