Submergence weaves together two disparate worlds through the lives of James More and Danielle Flinders. James, an Englishman held captive by jihadist fighters in Africa, is haunted by memories of a past love affair. Meanwhile, Danielle, a biomathematician, is about to embark on a profound journey to the ocean floor. Their stories converge as they reflect on their intense romance from the Christmas before, exploring themes of love, science, and existential threats in a rapidly changing world.