In a room with no windows on the coast of Africa, an Englishman, James More, is held captive by jihadist fighters. Posing as a water expert to report for the Secret Intelligence Service on al-Qaeda activity in the area, he now faces extreme privation, mock executions, and forced marches through the arid badlands of Somalia.

Thousands of miles away on the Greenland Sea, Danielle Flinders, a biomathematician, half-French, half-Australian, prepares to dive in a submersible to the ocean floor. She is obsessed with the life that multiplies in the darkness of the lowest strata of water.

In their confines they are drawn back to the Christmas of the previous year, and to a French hotel on the Atlantic coast, where a chance encounter on the beach led to an intense and enduring romance. James, a descendant of Thomas More, escapes to utopias both imagined and remembered, to fragments of his life before his incarceration, to books read, to paintings and music that haunt him now. Danny is drawn back to beginnings: to mythical and scientific origins, and to her own. It is to each other and to the oceans that they both most frequently return: magnetic and otherworldly, a comfort and a threat.