The Thing-Thing saga continues! The object of the game is kill or be killed. It's a fight for survival through the ranks of the Systems Corp. Mercenaries as well as any other horrors their rampant genetic experimentation have created.
All of the Systems Corp. facilities are on lockdown and all of the doors in your path have been locked. You'll have to fight through the requisite enemies in order to move on.
For the most part Systems Corp. Mercenaries teleport their way to you behind a locked door. Don't get pinned between them, and don't let them get too close!
After you've killed enough enemies you'll get a message on the bottom of your screen saying either "New Area Open" or "Exit Open" depending on the area. From here you can continue to fight or move on to the next area.