Chapter 1 - Polus

The crew members go to complete an mission on a recently collinated planet that was named Polus. As they do though they attract a creature which kills and takes form as one of the crew members. For the sake if the theory let's say they killed no one.

Chapter 2 - The Skeld

The creature is now on board with the crew and is arriving home with them. They start killing and get innocents thrown off the ship which convince the other crew they are not what they are. They then stop so they can arrive at Mira HQ.

Chapter 3 - Mira HQ

The crew arrive back on Earth to their HQ and rest to find the next day it's broken. They now have to fix it which leads the impostor to kill them all and have other impostors take their forms so they can bring more of them back to take over earth and eventually the galaxy.

That's all I got till a new map comes out.