Sisters Lisa (Mandy Moore) and Kate (Claire Holt) are on vacation in Mexico after Lisa's boyfriend recently broke up with her. In an attempt to cheer her up, Kate takes her out drinking and dancing, where they meet two local men who invite them to go and watch sharks from a diving cage. Lisa hesitates, but Kate argues it would show Lisa's ex-boyfriend that she isn't boring and would make him regret breaking up with her.
Four teens explore a ruined underwater city and quickly find themselves in a watery hell as their adventure turns to horror. As they swim deeper into the claustrophobic labyrinth of caves, they enter the territory of the deadliest shark species in the ocean.