Lisa (Mandy Moore) and Kate (Claire Holt) are two sisters who spend their holiday in Mexico. Lisa’s girlfriend left her, motivating the fact that she turned the relationship into something boring. Kate tries to cheer her sister by taking her to the dance and spending it. They then go to a dive site to see the sharks closely, and Captain Taylor mourns the sharks. Lisa is skeptical about this adventure, but Taylor assures her that there’s nothing wrong with her.
Several locals sink before them and after some time they return to the surface and are unharmed. Captain Taylor sends the two sisters to the cage, but their misfortune makes the thread that holds the cage tear and the sisters reach the bottom of the sea – 47 meters deep. This makes the communication system no longer work, coming out of its range. Kate swims up to a depth of 40 meters to get into the reach of the communication system, and Taylor makes sure Javier (Chris J. Johnson) comes after them with a spare cable.
He also advises the girls to stay in the cage because the sharks are all around them. Girls start to panic when oxygen reaches the end. They are waiting in the cage for Javier’s arrival, but they notice a torch at a distance, and, thinking that Javier can not see them, Lisa tries to go to him because Kate’s oxygen supply is lower.