Since some people have started posting their theories, I felt that I should throw in my two cents.

So, the story starts off similar to most of the other theories, Hitler needs more troops since he knows the war is lost. So he commisions Dr. Maxis, one of the most intelligent scientist in Germany at the time to build him an undead army. So another project is added to Germany's wunderwaffle program, codenamed Group 935. They are given the research facility of Der Riese to conduct their experiments. Unforunatly, for Maxis the experiments go nowhere and Hitler is considering to divert money to another project. Enter Dr. Edward Richtofen, a genius whose work in the mysterious Element 115 have made him a legend in Germany. Maxis quickly signs him on as an assistant. With the inclusion of element 115 the experiment becomes a success and begins to branch out. They are given the asylum of Verruckt to conduct experiments on the insane. With the sucess of the zombies, Maxis begins to work on other projects including a teleportation system, all powered by 115. It is around this time that his daughter Samantha asks for a pet dog, maxis sees this as a oppurtonity to test element 115's reaction on animals as well as give a test run to his teleporters. And so Samantha is given a Dog named fluffy.

Maxis begins to run experiments on Fluffy and her puppies and begins to treat edward more like in idiot than fellow scientist. This drives Edward to contact the Illuminati an scientific secret society who tell him to kill Maxis and give control of the project to them. So, Edward locks Maxis and Samantha in the teleportation chamber and (supposedly) kills them. Unforunatly, the effect of this action causes Edward to go insane and causes changes his personality that result in the Richtofen we meet in the game. With out Maxis around the whole place pretty much goes to h***. The zombies escape and begin to kill the workers involved in the project. Richtofen also escapes and heads toward Germany's ally Japan, where he hears that there has been a discovery of element 115. He reaches the village of Shi no numa and begins to design the Wunderwuffle DG-2 using the meteor he finds there. This tampering leads to infecting the whole village which sets the scene for Shi no numa.

Mean while in Europe a small unit of American and Russian troops stumble upon the Verruckt facility and are forced to fight the insane undead. They steal a supply plane to escape and feel they are safe until a zombie on board forces them to crash in a nearby Airfield. There they form a last stand against the zombies before eventually all dieing to the undead horde. Fortunatly, for the world they are able to send out a message to their respective governments warning them about the threat before their death. This leads the American and government to send their war heros, Tank Demsey and the Russian drunk Nicholai along with a Japanese POW (Takeo) to find the last remaining link to the project, Richtofen, in Shi no numa. After reaching the destination they join forces with the mad doctor to combat the Undead natives. After clearing the village, Richtofen teleports them to Der Riese, where they attempt to finish the fight against the undead using devices like the pack-a-punch and perks. However, being outmanned and outgunned they most likely perish in their attempt leaving the world to combat the undead threat alone.... or do they???

Well there you go my take on things.. Sorry for the length. Tell me what you think.