The game revolves around a young man named Daniel Garner, who is happily married to his wife Catherine. At the start of the game, Daniel is about to take Catherine out for a birthday meal. As they drive towards their destination at high speed, in the pouring rain, Daniel takes his eyes off the road to look at his wife and while his attention is diverted, he ploughs their car into a truck. Both of them are killed instantly. While Catherine manages to make it to Heaven and lives in harmony, Daniel is trapped in Purgatory and is told that in order to receive purification, he has to kill four of Lucifer’s generals in order to prevent a war between Heaven and Hell. With little choice available to him Daniel accepts the task. With the forces of Hell seizing and corrupting various parts of Purgatory, Daniel goes through different and random phases of history within Purgatory, which range greatly from ancient times to the modern era. These include Medieval Europe, Babylon, the Crusades, 1800s and even a time slice of modern industry.

Painkiller has 5 guns, each with two normal modes of fire and often a third combination fire. Painkiller — The eponymous Painkiller is a mechanized close combat weapon. The primary fire (Pain) causes the end of the weapon to open up into a rapidly-rotating fan of blades. The secondary fire (Killer) ejects the blades from the weapon and can damage enemies and also attach to inanimate objects. A damaging beam of energy then projects from the player to the point of impact, provided the player aims the weapon towards the disengaged blades. Pressing the secondary fire button again will return the blades to the weapon. The combination fire is performed by holding down the primary fire, then hitting the secondary fire, causing the rotating blades to be fired from the weapon. It is also used to ‘painkiller jump’, which involves standing on a piece of debris and using secondary fire on it in order to launch yourself into the air. Also, hitting a defeated enemy multiple times with the Painkiller’s secondary fire will cause them to drop trinkets, which can be picked up to add to your total gold score for that level. “Gibbing” the enemy while they are suspended in mid-air, either by utilizing the Painkiller weapon’s primary mode or one of the other weapons, will drop a valuable gold ring, also adding to the player’s gold score. This has given rise to the practice known as “Monster Juggling” using the Painkiller secondary mode repeatedly to keep the corpse suspended in mid air and drop gold trinkets, then finishing it off with a short burst in the primary mode to reduce it to gibs and drop a golden ring, all of which must be accomplished before the corpse dissolves into nothingness a few seconds after its death. This is also a potent weapon in deathmatch and other multiplayer games, as it allows you to sweep in for the quick kill and confuse opponents.Shotgun — The primary mode fires two shotgun shells, the closer the target, the more devastating result will be achieved but the farther the target is away, the wider the pellet spread will be. The shotgun’s secondary fire freezes opponents solid for a short time. After they’re frozen, damage from any source will destroy the enemy.Stakegun — The stakegun is a pneumatic sling that shoots stakes at the enemy that can pin their dead bodies to the wall or rarely to other enemies. Its secondary fire is a grenade launcher and if the bouncing grenade is hit by a stake, it becomes a fast-moving explosive missile. The stakes and grenades launched fly in an arc, and the stakes erupt in fire if in the air long enough. Grenade can also be shot with a stake while it’s still in air. That way, grenade achieves much more range and explodes on impact when it hits anything, not only enemies. In deathmatch games, impaling an opponent with a stake can lead to an instant death.Chaingun — The chaingun’s primary fire is a rocket launcher and the secondary fire is what most will expect from a rapid-firing chaingun. This combo is used extensively in deathmatch games, for its ability to send players flying. The chaingun does extra damage to opponents and monsters when they are in the air.Electrodriver — The electrodriver’s primary fire launches shuriken at enemies. The secondary fire shoots a stream of electricity at close range. The combination fire is performed by holding down the secondary fire, then hitting the primary fire, which launches the electro driver disk which damages any enemies in close proximity to it with electricity.

from wikipedia.org