On a boat heading out into the North Sea, Ellen Brooke steels herself to spend almost a month locked inside a hyperbaric chamber with five other divers. They are all being paid handsomely for this work – to be lowered each day inside a diving bell to the sea bed, taking it in turns to dive down and repair oil pipes that lie in the dark waters. It is a close knit team and it has to be: any error or loss of trust could be catastrophic. All is going to plan until one of the divers is found unresponsive in his bunk. He hadn’t left the chamber. It will take four more days of decompression, locked away together, before the hatch can be opened. Four more days of bare steel, intrusive thoughts, and the constant struggle not to give way to panic. Mind games, exhaustion, suspicion, and, most of all, pressure. And if someone does unlock the door, everyone dies.