HIGH PRESSURE OUTSIDE. On a boat heading out into the North Sea, Ellen Brooke steels herself to spend almost a month working underwater with five other divers. It is a close knit team and it has to be: any error or loss of trust in the living quarters could be catastrophic. EXTREME PRESSURE INSIDE. 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 wondering if one among you is a killer.

The constant struggle not to give way to panic. Because if someone does unlock the door, everyone dies...