Final Doom

In TNT: Evilution the UAC once again are intent on developing and experimenting with dimensional gateway technology. They set up a base on Io, one of the moons of Jupiter, with a solid detachment of space marines for protection. The marines do their job well: when the first experimental gateway is opened they annihilate the forces of Hell. Research continues with more confidence and all security measures turned at the gateway.

A few months later, the yearly supply ship comes ahead of schedule, and looks strange and unusually big on the radar. The lax radar operators decide that there is nothing to worry about... The personnel of the base go out to behold the terrible truth: it is a spaceship from Hell, built of steel, stone, flesh, bone and corruption. The ship's enormous gates open to unleash a rain of demons on the base. Quickly, the whole facility is overrun, and everyone is slain or zombified.

The main character, the nameless space marine (who was revealed to be the marine commander on the moon) has been away on a walk at that time and thus escapes death or zombification. After being attacked by an imp, he rushes back to the base where he sees the demonic spaceship still hovering above it and realizes what has happened. Then he swears that he would avenge his slain troops and sets out to kill as many demons as possible.

In the end, the marine defeated the "demon spitter" and the game describes "something rumbles in the distance. A blue light glows within the ruined skull of the demon-spitter.