Instead of calling the Colonial Marines in Alien II, somebody dials the wrong number. And when Ripley wakes up on her way to LV426? The first thing she sees are hulking humanoids with plugs all over the place, and where there are no plugs, there are scars.
"Is she a Heretic?" asks one.
"No brother, leave her be" answers another. "She may have experience relevant to our mission."
In all there are the same number of Space Marines as there would have been Colonial Marines, and these guys are Grey Knights(which probably explains why they got the job...)
"Wait, who are you people exactly?" Ripley asks.
The answer comes stoically; "We are the warriors of the Grey Knights, armoured in faith, shielded by devotion and armed with purity of purpose. But greater even than these, we carry the light of the divine Emperor of Man into the dark places to purge the Daemonic wherever it may be found."
"Oh...kay."
Needless to say, this gets a little interesting.
What happens on LV426?
AND...will Ripley embrace The Emperor as her lord and saviour?
***Scenario 2: Swap out the Grey Knights for ANGRY MARINES...