Six experienced saturation divers are locked inside a hyperbaric chamber. Calm and professional, they know that rapid decompression would be fatal and so they work in shifts, breathing helium, and surviving in hot, close quarters.

Then one of them is found dead in his bunk.

With four days of decompression to go before the locked hatch to the chamber can be safely opened, the group must watch one another's backs at all times. And when another diver is discovered unresponsive, everyone is on edge. What...or who...is taking them out one by one? And will any of them still be alive by the time the four days is up or will paranoia, exhaustion, suspicion, and pressure destroy them all?