Set in a giant salt mine, amidst the workings of an alien spacecraft, and in the air over Germany, Ünter continues the epic story of Hitler's final roll of the dice. After the events of Forever, an alien chrono-device remains hidden within the Alps in a secret facility known as Der Dom. To secure it, Sgt. Jim Thompson leads a daring airborne operation deep into Nazi Germany in October 1944. Yanis Miller, the Israeli mathematician who risked everything to warn MI5 and was dismissed as a fantastical crank, and his partner, unflappable Inspector Jillian Qualmes, find themselves caught between a stubborn machine with illegible symbols and an SS Division poised to eradicate the paratroopers from German soil. While Thompson's men are tasked with defending the mountain, Yanis and Jillian race against time to find a way out.

But Sturmbannführer Kristock Keck has managed to travel back and forth through time and briefly exist as two people at once. Captured but defiant, he plots a bloody escape, determined to lead the SS to victory in the mine's heavily defended galleries. Meanwhile, his former SS colleague, sadistic assassin Inge Weber, employs her unique talents while embedded on a U-boat with the Kriegsmarine. But will her personal agenda take priority over the common cause: creating the Fourth Reich?

In the second book of the Forever trilogy, an international contingent from past and future must fight shoulder to shoulder, all while the Nazis hurry to prepare the most terrible and lethal response. Thompson's men know that if they fail, an Axis victory in Europe will surely follow.