Plot
The UAC has been conducting experiments on demons, again.
They've lost contact with the base where they were conducting the tests.
What a shock.
Your job is to go in and clear out the base.
Alas, a failsafe means that the exit from the complex cannot be accessed until the full test regimen has been completed.
So you're ALSO going to need to make sure the tests are done.
Don't let the test subject die, or your only way out is as chunky giblets.