1958. France’s first nuclear submarine, Plongeur, leaves port for the first of its sea trials. On board, gathered together for the first time, one of the navy’s most experienced captains and a tiny skeleton crew of sailors, engineers and scientists. The sub makes her first dive and goes down, and down and down. Out of control, it plummets to a depth where the pressure will crush her hull, killing everyone on board, and beyond. The pressure builds, the hull protests, the crew prepare for death, the boat reaches the bottom of the sea and finds – nothing. Her final dive continues, the pressure begins to relent, but the depth gauge is useless. They have gone miles down. Hundreds of miles, thousands. And so it goes on. On board, the crew succumb to madness, betrayal, religious mania and murder. Has the Plongeur left the limits of our world and gone elsewhere?