cause the moon to open up they had just solved the answer to the ultimate mathematical question on the moon,which is actually not a moon,but in fact....
They ended up teaming up with the survivors from left 4 dead,and they gave the new characters,known as the Men of Math,and they all grabbed guns and they....
Needed to find shelter from them so they found an impenetrable zombie fortress that could only be entered from the front door! But there were more zombies so the friends had to...