The action feels disjointed through the precisely first 30 minutes, as it shows the characters playing Live Action Role Playing Game (LARPG). The next 30 min are used to build tension, without anything really important happening, and exactly at 60 min of film, the killing starts. The whole film ends in 90 min, plus 5 min of credits.
The game's scenario is "what if Hitler had won WWII," which would have led to a total nuclear war, and although the Nazis would have won WWIII, the survivors would be forced to live in the titular bunker. "What if" stories are not uncommon in RPG (Live Action or conventional "table top"), as it's a genre in Science Fiction and Fantasy (see Philip K. Dick's "The Man in The High Castle" for a very similar scenario.)
Some direction choices make those first 30 min very confusing if the viewer is not paying close attention (there's no kind of hint, visual or otherwise, to acknowledge when the actors are playing their parts or it's their characters playing the game, or, if there is, it was too subtle to me that I didn't notice.)
Suddenly, a power shortage forces the players out of the bunker, but some are left behind for reasons. Then, an "invisible force" locks them inside, also for reasons.
The actual story is a bit confusing, so here are the main points:
1) A woman is pregnant and reveals that to her boyfriend, after they make snu-snu in a bunker's restroom.
2) The boyfriend wants the baby, but the woman doesn't (she thinks they are not "parent material.")
3) A woman's cousin is revealed to be in love with her. She obviously rejects the idea.
4) The woman starts having visions of a dead woman, which sympathised with the Nazis to survive. That woman was also pregnant and, during an attack, has her belly pierced by a glass shard and dies.
5) A ghost (it's not clear if the dead woman's or someone else's) kills a random character (the one trying to operate an old radio, to call for help.) No apparent reason is presented for her murder.
6) Another random character is killed by fire. Again, no apparent reason.
7) One character turns out to be a real Nazi, murderous and all, though with no consequences (except knocking the incestuous cousin down,) as the "invisible force" scares him to death immediately after the revelation. Again, no apparent reason (of course, unless one considers "being a Nazi" reason enough to kill a Nazi...)
8) The incestuous cousin wakes up and, in an attack of fury, kills a random character, before going after his beloved one.
9) The woman discovers that her boyfriend had been killed earlier (off screen) and deduces (correctly) that it's been her jealous cousin.
10) Rejected again, the cousin strangles the woman, but she's possessed and saved by the spirit of the dead woman. She/they kill(s) the cousin with a glass shard (which is shown piercing his belly, but on the next shot, it's stuck in the middle of his chest, supposedly accounting for his seemingly instant death.)
11) The possessed woman walks to the bunker's gates, which magically open. Credits roll.
The film has a good atmosphere and the actors do very fine (they overact when they are playing LARPG in the first 30 min, then go with more convincing tones after their characters find themselves locked in.)
The main problem is the plot. Too many questions have no answer (most because any answer would make no sense.)
For example, why were the gates locked? If it was to keep the pregnant woman inside, so the ghost woman could possess her, why risk letting the majority of the players get out? It was just by a plot device that she didn't get out with them. If it was another ghost, why the gates only opened after the woman was possessed? Maybe she was being helped by her Nazi lover. In that cause, in there was a Nazi ghost (which is never hinted), why didn't it try to possess someone else in the bunker (or maybe it did, the woman was pregnant, it could have possessed the , what is also never hinted.)
It's worth one view, mainly for technicals (camerawork, sound effects, props, lighting), but it won't stand the test of time.