Based on J.M. Ledgard's book of the same name, "Submergence" focuses on two lovers who are separated by thousands of miles and use the memories of their intense romance to help them get through the life-or-death situations they're facing in their present lives.

James Moore, enduring captivity by jihadist fighters in Somalia, and Danielle Flinders, exploring the depths of the ocean floor, are desperate to reunite again after their chance encounter on a beach on the Atlantic coast the previous Christmas.