The problem your protagonist has is that, when the adventure begins, he starts having memories of his childhood that make him doubt which band he should be helping. And as you advance through the levels, you too will begin to question which side you're on.
Gameplay, on the other hand, is quite simple. You can move your character in all directions and shoot at any enemies you find; they usually go down after a bullet or two. Some of the weapons you have access to include a pistol, shotgun, Desert Eagle, and even a Claymore mine.
In addition to weapons, the levels are stocked with other items like first-aid kits, bulletproof vests, and keys, which you'll need to open locked doors.
Valkyrie is an action game that, in addition to good, direct gameplay, has a truly intriguing story that provides the player with a difficult dilemma. If the story doesn't interest you, well, you can always just shoot.