"In the Deep" concerns sisters Lisa (Moore) and Kate (Holt), who set off for a fun-filled vacation in Mexico. When offered the chance to go cage-diving in an area populated by magnificent great white sharks, Kate is quick to seize the opportunity. After some prodding, she also manages to convince her sister to come along for the ride. Unfortunately, their trip soon becomes a desperate fight for survival, as the cage they're in breaks free from the boat it's attached to and plummets to the ocean floor.
With an oxygen supply of less than an hour to work with, the stranded sisters must find a way to get back to the safety of the boat waiting above, all the while navigating through some 47 meters of water inhabited by the fearsome oceanic predators the duo originally looked forward to seeing in person.