What is the story to Nazi Zombies (read details)?

I have been recently interested the Call of Duty Nazi zombies series story. I am not going to be a fag and look up the whole thing on the internet, i just want to know about the beggining so i have a place to start. Can somebody please explain the beggining (by the beggining i mean up to der reise) in detail please i will best answer the answer with the most "clues". Thanks