As for story: you will spawn to a remote UAC MARS Facility, as soon as you arrive you notice that the Facility is in unstable state (first major area). You have Heavy Assault Rifle with Sniper Mod, A Shotgun with Triple Shot mod and Grenades in use. Naturally your first objective is to find the exit. But there are some hiccups on the way and your objective changes on the flow: to reach exit there are some obstacles ahead. You will be provided feedback via audio messages and mission objectives. The difficulty of this mission is moderate due checkpoint system.
When creating this map, I didn't want to create a completely narrative story, but a traditional Doom 1 like action packed experience. The progression is somewhat straight forward and you will be provided with some logical hints to complete the mission objectives. Some of the resources and more powerful weapons are behind "secret locations" you'll need to find. So keep your eyes pealed on Armor Shards and Medium Healt lootables - they will guide you to right direction.
There are some additional mechanics included. For example: if you manage to pick up BFG, don't toss it away - you will need it to complete the mission and you will be prompted about it: the demon difficulty is greatly increased on the following areas. Also if you speedrun the map not daring to kill everything that moves -- it means you are a causal gamer which renders some arenas bit easier with less simultaneous enemy spawns. So your actions do have consequences.
Upon death you can continue the fight from where you left it and you can keep your arsenal. To make sure you can have fun and beat the mission you have unlimited lives and this is the point where you can set free the Carnage!
The map consist of two major areas: the first one is easier and the second one is bit more challenging. Here is the map layout: