A retro, Doom/Quake-style FPS (with a pinch of Half-Life enviromental storytelling) featuring you, the player, as a nameless soldier stuck in a massive, high-tech Nazi fortress. It's got all the usual stuff: cool weapons, secrets, hordes of enemies, but there's also various upgrades for your weapons in hidden spots (explosive shots, short power-ups for killing enemies, etc.), though each weapon can only have one upgrade at a time.
There are also four enemy "factions", each with their own leader, who contacts you throughout the game and is eventually faced in a boss fight:
1. The personnel of the base, basic Nazi foot soldiers led by a general (who's also the base's commander).
2. An army of cybernetically and genetically enhanced soldiers, plus Nazi robots, led by a mad scientist.
3. The Black Sun Cabal: a spooky cult dabbling in the occult: demon summoning, necromancy, dark magic, you name it. They are led by some shadowy priest dude.
4. Demons - monstrous beings from another dimension, summoned by the Black Sun Division. They don't really have a leader, with the only thing close enough to fulfill the role being the giant god-like demon who serves as the final boss.
One of the plotpoints is that the demons are fiercely protective of each other, which comes in partway through the game after you kill some of them. As a result, the rest chimp out and turn on the other factions, though they are quickly joined by the Black Sun Cabal, which worships them. Thus, the rest of the game is a 3-way battle between you, the Nazis + the Scientist's army and the Cabal + the demons.