1941. A German submarine, U-330, patrols the stormy, inhospitable waters of the north Atlantic. It is commanded by Siegfried Lorenz, a maverick SS officer who does not believe in the war he is bound by duty and honour to fight.

U-330 receives instructions to collect two prisoners from a vessel located off the Icelandic coast, and a British submarine commander, Lawrence Sutherland, and a Norwegian academic, Professor Bjørnar Grimstad, are taken on board. Contact between the prisoners and Lorenz is forbidden, and it transpires that this secret mission has been ordered from high up in the SS. It is rumoured that Grimstad is working on a secret weapon that could change the course of the war . . .

Then Sutherland goes rogue, and a series of shocking, brutal events occur. In the aftermath, disturbing things start happening on the boat - it seems that a lethal, supernatural force is stalking the crew, wrestling with Lorenz for control. A thousand feet under the dark, icy waves, it doesn't matter how loud you scream . . .