17-year-old Josh McGuire (Rhys Wakefield), expedition bank-roller Carl Hurley (Ioan Gruffudd) and his girlfriend, Victoria (Alice Parkinson), travel to the Esa'ala Cave, an underwater cave exploration site in Papua New Guinea. Josh's father Frank (Richard Roxburgh), a master diver, has already established a forward base camp at a lower level inside the cave, where the team have been exploring for months. The relationship between father and son is distant and strained. Josh is resentful of his father's forceful and uncompromising nature and Frank is equally disapproving of what he sees as his son's lack of discipline.
While exploring a section of the cave known as Devil's Restriction, Frank and diver Judes (Allison Cratchley) meet with an accident which results in Judes' death. The remaining dive team witness the event on the rover's remote camera, fueling agruments and speculation among the group and further straining the relationship between Josh and his father.
Frank orders his son and a portion of the dive team to return to the surface unaware that a fierce storm has developed, and all attempts by the surface team to warn the divers has failed. Josh and fellow diver, Luko discover that communications with the surface have broken down and return below to warn the others of the impending danger. During their descent, a portion of the cave system collapses with the force of the rushing water, trapping the remaining divers below. Realizing that any chance of rescue from the surface is futile, and with the water level quickly rising, the group must now face a life or death struggle for survival as they attempt to find a route through the unexplored depths of the cavern.