The story so far

It starts in World at War. A crew from a B-52 bomber crashes somewhere in Germany. As the crew wakes up, they notice a lot of people shambling toward them. As their vision clears, they realize that they are zombies. They barricade themselves in a nearby abandoned building and fight for their lives. This is the first map that came with World at War. It is unknown if the crew survives, or they are eventually overrun. Probably overrun.
Next, a detachment of marines comes across an insane asylum. Tank Dempsy is believed to be with them, although that is up for debate. The surrounding forest is on fire, likely set by nearby villagers in order to stop the zombies. It didn't work. The marines are immediately attacked by the corpses in the asylum. They find evidence that experiments were performed here, both on the living at the dead. (One of the four rooms you teleport to in Kino is actually in the asylum, before it was overrun. It's the room with the chair.) A handful of the marines survive and escape. This is the second map in World at War.
Japan, Russia, and the U.S. realize that the zombies are a threat, so Dempsy is sent with Nikoli and Takaeo to investigate a swamp where it is believed the experiments were moved. When they arrive, they find Edward Richtofen. A radio message to Peter (the hanging corpse) reveals that the situation at the asylum was no able to be contained, so the experiments were moved to this swamp. Richtofen hints that he knows what's going on, but never explains. The four soldiers are forced to fight their way out of the swamp. It is suggested that, just before they are overwhelmed, Richtofen teleports them all to safety. This is the third map in World at War.
The men teleport to Der Rise, where the experiments were started. Here, it is revealed that the zombies are created with an element called 115. Richtofen worked under a Doctor Ludvig Maxis. Maxis had a daughter named Samantha. The Hell Hounds come from Sam's dog Fluffy, who Maxis sent through the teleporter, mutating it. In another radio message, it is revealed that Richtofen betrays Maxis and locks him and his daughter in a room with a Hell Hound. It is believed they are both killed, as you can find their blood splatter on a wall in the map. However, Samantha may still be alive, or she is a ghost out for revenge. She is the one who controls the box (she loves teddy bears) and is the little girl/demon voice that says "Max Ammo" and other things. The four men fight at Der Rise, but are eventually backed into a corner, seemingly to their deaths. This is the final map in World at War.
In Black Ops, the comic reveals what happened. The teleport malfunctioned and launched the four into the future just before they were killed. Richtofen's message in single player explains a lot, and also suggests that Sam is still alive, just in another time. However, the rooms that you are teleported to seem to imply that Sam is actually playing a game with the four men, as she has a mini map of Der Rise with the four figures in it. More radio messages reveal that Maxis was actually trying to mind control the zombies and use them for weapons. It didn't work. The battle to stop the outbreak rages on. 
"Five" map isn't connected officially yet, but there are little hints that suggest that the U.S. knows about the zombies and the teleporters. Whether the U.S. is preparing to fight the zombies, or attempting to recreate them is no clear yet. However, something has gone wrong, as the zombies are now attacking the Pentagon.

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