This mission takes place on paradise, that is to say, Earth. Alien shape changers have invaded on All Hallows Eve.
The population are all out celebrating, everyone is in costume and partying the night away. The aliens are trying to blend into the populace before attacking. They can easily appear as costumed humans or Policemen or even members of the 3:16th, but there is always "something" not quite right which the wary can usually spot (NFA). When revealed they change to nightmare like creatures (often drawing images from the people around them) and attack.