The very basic idea is that you play as some random guy in a white coat and kill other random people. Along the way you'll collect weapons. You recieive ammo in the universal form of BloodCoins, which enemies drop when you kill them.
The game starts with your character going down an escalator (or a lift, haven't decided) that goes down into the Earth by accident. There you'll find the tutorial; Hell. From there, you go back up the escalator to ground level, where you'll find some people who are very messed up, so you have to get into their heads and sort it out. Once you've done them all, you'll be able to use another escalator which goes into the sky, therefore completing the game.
Controls:
Up - Jump
Left/Right - Lateral movement
Mouse - Aim, shoot.