Lena, a seasoned Nazi-hunter, tails Neurath, the former SS Gruppenführer who worked with the war-criminal Klausener during World War II, to Paraguay. During interrogation, Neurath taunts her about an active cabal of Nazi loyalists and dies of a heart attack. In his apartment she uncovers a map of the former Yugoslavia, papers concerning the British engineer Hunt, and a ring with a retractable key embedded in the signet.
Meanwhile, the same team of mercenaries who arrived at the end of the first film breach the outpost and move into the complex. An elderly Klausener watches via a helmet-cam as they are promptly attacked and slaughtered by a battalion of zombie Waffen-SS troops, led by SS Brigadeführer Götz.
In the former Yugoslavia, Lena runs into Wallace, a physicist seeking truth about the unified field theory; he believes Hunt and his team are dead and that no one should control what they were searching for, Die Glocke. He explains that a NATO rapid-reaction force has been secretly battling the undead SS guards, who have been slaughtering villagers across the region and inflicting heavy casualties on NATO troops. According to a contact in a local militia group, a reality-bending electromagnetic field has been steadily expanding across the countryside with the source at the outpost bunker.
Lena and Wallace trek to the militia camp, only to find it under attack from the SS, with the militiamen and NATO soldiers stationed there slaughtered, including Wallace's contact. Hiding under a soldier's corpse, Lena witnesses an SS-Mann brutally kill a militiaman. The next morning, they discover NATO forces failed to set off an EMP to disable the machine protecting the SS guards. Continuing through the countryside they observe devastated villages. Wallace explains his knowledge from his journal of runic symbols and history about Black Sun—the Wunderwaffe group responsible for creating Die Glocke. They run into an undead SS group but are saved by a British SAS fireteam led by Macavoy and Abbot, with Carlisle and Hall and an engineer; the latter recognises Wallace but dies in a skirmish while setting up an EMP to disable a small area of the surrounding electromagnetic field, allowing the SAS to permanently kill the attacking SS troopers. The British enlist Wallace as their engineer and [Macavoy] orders Lena to leave them until reinforcements arrive.
Lena follows them and helps them survive another SS attack. They begrudgingly allow her to join, explaining their mission is to secure Die Glocke before a tactical nuclear strike is launched on the bunker. They reach the outpost and trace the cabling to reach Die Glocke's engine room, but discover the machine has been moved. Hunt's documents and equations are graffitied on the walls, leading Wallace to deduce Klausner's people hired Hunt to retrieve the device. The soldiers are attacked by an SS guard and [Macavoy] is killed. Wallace determines Die Glocke was moved deeper into the bunker via a lift hidden behind the chamber's forward wall; [Abbot], now in command, orders Hall to set-up flares giving the all-clear for the NATO strike once they evacuate.
The lift takes them underground where they find bodies of locals and mercenaries dumped in piles by the SS, led by an undead SS matron. [Carlisle] distracts them and saves Lena from being discovered, but he is stabbed to death by the matron. Wallace locates the control schematics and points them to the containment chamber; [Abbot] and Lena find Hunt's body hardwired to Die Glocke's large generator that controls the undead SS; Götz has managed to increase the generator's electro-magnetic emission range, enabling his battalion to expand beyond the bunker's initial containment field. [Abbot] is killed in an ambush while Hall is caught in a struggle with an SS-Mann and accidentally sets off the flares; with communications down via the EMP field, they are unable to call-off the launch. Wallace manages to trip an EMP wave that temporarily leaves all nearby SS guards vulnerable, allowing Hall to kill the matron and two other guards.
Hunt, barely alive, reveals the machine's unified field core device could only be powered down by Black Sun personnel who had the operating keys. Lena produces Neurath's ring with the key to deactivate the core but is stopped by Götz. Wallace is saved by Hall and they go to find the others. Götz prepares to inject Lena but Hunt uses his last remaining life force to discharge the core's containment energy into the reviving SS platoon. Lena grabs the key and shuts down the waves of esotericism energy containing the guards; Götz manages to hold off Hunt's attack but is beaten and his skull smashed by Lena, destroying him. She retrieves Die Glocke's field containment core and rendezvous with Wallace and Hall in the lift. Wallace suddenly executes Hall and wounds Lena, leaving her to die and escapes just before the outpost is bombed.
Sometime later, Wallace meets Klausner with one half the core as a bargaining chip in case he is double-crossed, whilst holding the other half at an auction. He suddenly receives a call from Lena, who survived the strike by hiding deep underground in the bunker; she warns him she is coming for him and Klausner.
