So, the Protagonist gets reincarnated in the 1920/30s, but he does not know that he is in Wolfenstein.
Thats it really, it would be really funny if he focuses on doing the typical protagonist stuff and then BAM, Wolfenstein.
If you don't know Wolfenstein, it's basically a game where you kill nazis. Also they have inhuman strength, also stole the tech that Jews used to create super war machines. (Look it up for yourself, it's hard to explain) With that, they won the second world war and the world went into chaos.
The game doesn't take itself too seriously mind you. You can find nuka cola and a skyrim helmet in the first act of old blood. The game is really fun tho.