Half-Life: Opposing Force

Half-Life: Opposing Force (Commonly referred to as Op4 or OpFor) is a critically acclaimed expansion pack for the popular first-person shooter Half-Life, developed by Gearbox Software and released by Valve Software on October 31, 1999. The expansion's single-player mode features the same setting as the original, with the twist that the player is cast not as Gordon Freeman, but as Corporal Adrian Shephard, a U.S. Marine. Shephard is sent into the huge underground Black Mesa Research Facility on an undisclosed mission, but things go wrong and he finds himself fighting for survival against sinister government agents and monstrous aliens.

Plot
The action of Opposing Force cleverly interweaves with the events of Half-Life, so that the player catches a glimpse of Gordon Freeman, the player character from the original game, sees an officer making a pivotal radio broadcast that Freeman overhears in the first game, and visits some recognizable locations.

In the original Half-Life, a "resonance cascade" opens an extra-dimensional rift, which cause Xen aliens to teleport into the Black Mesa Research Facility. The surviving scientists and security guards at the facility struggle to escape. Responding swiftly, the United States military initiates a massive "clean-up" operation to eradicate the alien presence at Black Mesa as well as "silence" any witnesses. As the game begins, Shephard is part of this operation. After his transport aircraft is shot down and he is cut off from the rest of his unit, the clean-up mission is abandoned and Shephard joins forces with the scattered survivors seeking escape.

As the tide of the furious battle begins to favor the alien army, Shephard realizes the military forces have been ordered to pull out. The Black Op forces (seen only briefly in the first game) are acting independently of the marines, and intend to destroy the entire complex with a thermonuclear warhead. Shephard deactivates the bomb but later sees the G-Man reactivating it. After defeating a monstrous boss alien, Shephard finds himself back in a V-22 Osprey transport, similar to the one he was on at the beginning of the game, with the G-Man. The nuke at Black Mesa detonates and the G-Man detains Shephard someplace where he can do no harm and cannot be harmed, pending further evaluation...

Chapters

 * Opposing Force - As Corporal Adrian Shephard and his squad are approaching the LZ (Landing Zone 112) at the Black Mesa Research Facility, his plane (codenamed Goose 7) is shot down by Xen Manta Rays (aka Flyers), along with several other V-22 Ospreys (first one is codenamed Goose 3). The rest of the opening sequence consists of Shephard continually losing and regaining consciousness, seeing conflict between the Xen aliens and the Marines who survived the crash.


 * Welcome to Black Mesa- Shephard wakes up in an infirmary, not too far off from the crash site. He leaves the infirmary, learning that the other Marines who had survived the crash had been killed by the aliens. He makes his way towards the extraction point, where he is told via radio that the military forces are pulling out of the facility.


 * We Are Pulling Out- Shephard ventures through Black Mesa towards his assigned extraction point, and has first eye contact with the mysterious Race X that seems to be collecting humans for study in their world. Shephard reaches the extraction point, but is deliberately left behind due to the G-Man's actions.


 * Missing in Action- Shephard continues through Black Mesa, trying to find another way out after being abandoned at the extraction point. After traveling through a zombie-infested blast furnace, Shephard meets several other Marines from his company who have also been left behind. He joins forces with them as they try to escape from the facility.


 * Friendly Fire- After battling through an office complex filled with aliens, Shephard comes to a hangar where he sees two soldiers in black combat gear loading a nuclear warhead into the back of a truck. When they see him they attack, and Shephard is forced to kill them in self-defense. He realizes that the Black Ops soldiers have been sent in to kill the remaining HECU Marines and cover up the Black Mesa incident. Shephard meets up with several other surviving marines and fights the Black Ops, eventually reaching an underground railway system that leads to the Lambda complex.


 * We Are Not Alone- Shephard arrives at the Lambda complex just in time to see Gordon Freeman teleport to Xen, a scene from the first game. Shephard is also forced to travel through Xen to find a way out of the Lambda complex.


 * Crush Depth- Back in Black Mesa, Shephard arrives in a complex of underwater laboratories where several Ichthyosaurs are being held. After making his way through the labs, Shephard is again forced to teleport to Xen and back again to reach a new part of Black Mesa.


 * Vicarious Reality- Shephard fights his way through a part of Black Mesa featuring massive terrariums used to house Xen fauna being studied by the scientists. Eventually he reaches an older industrial area, and picks up a radio transmission from a group of Marines fighting some kind of massive creature.


 * Pit Worm's Nest- Shephard arrives in an industrial waste-processing area of Black Mesa, where an enormous Pit Worm is blocking his progress. Working his way through the surrounding area (littered with the corpses of the Marines who sent the distress signal in the previous chapter), Shephard manages to dump toxic waste on the Pit Worm, killing it and allowing him to access the next area (the sewers of Black Mesa). This chapter is very similar to the Blast Pit chapter from the original Half-Life.


 * Foxtrot Uniform- Accompanied by several fellow marines, Shephard makes his way through surface areas of Black Mesa controlled by Black Ops forces, including several snipers. He makes his way through the underground canals and sewers of Black Mesa, fighting through a Voltigore nest, and arrives at the hydroelectric dam featured in the first game. Here, a Gargantua has been trapped on top of the dam and several Marines are attempting to kill it. Shephard uses explosives to destroy the creature and leaves the area through the dam's exposed water pipes. This level is the equivalent of Surface Tension from the previous game.


 * The Package- Shephard fights his way across the surface areas of Black Mesa, facing down Black Ops troops, artillery, a black AH-64 Apache and alien Shock Troopers. Eventually he comes to an underground parking garage where the thermonuclear warhead from Friendly Fire chapter is being activated by two Black Ops commandos, apparently with the intent of destroying the facility. Shephard kills them and deactivates the nuke, but later sees the G-Man reactivating it.


 * Worlds Collide- After making his way through warehouses filled with Race X creatures and Black Ops soldiers fighting, Shephard arrives at an old industrial area of Black Mesa where an enormous Gene Worm is attempting to come through a portal from Xen. Shephard uses mounted laser weapons to blind and kill it; in its death throes, an enormous teleportation portal spreads outwards and envelops him.


 * Conclusion- Shephard regains consciousness inside a V-22 Osprey, confronted by the mysterious G-Man. As the G-Man talks to Shephard a white flash occurs in the distance, indicating that Black Mesa has been destroyed by the nuclear warhead. The G-Man then informs Shephard that he will be detained "in a place where you can do no possible harm, and where no harm can come to you." He explains this is done since Shepard reminds him of himself, but is still a dangerous witness. The G-Man walks into a portal, and the game then ends.

Weapons

 * Pipe wrench: The base mêlée weapon Shephard starts with, suitable for bashing crates and enemies alike. Alternate fire lets the player slowly prepare for a more powerful strike.


 * Combat knife: Another crude mêlée weapon that delivers only modest damage, but is very fast and effective against headcrabs or already wounded aliens. Stabbing an enemy in the back will result in a one-hit kill.


 * Barnacle (specimen 1176): The Black Mesa scientists were able to "tame" this Xen creature that normally adheres to ceilings. Its ability to latch onto distant organic targets (including enemies) using its long tongue makes it useful as a grappling hook allowing the player to scale heights and cross otherwise impassable gaps. Primary fire shoots out the tongue and pulls the player towards whatever it sticks to, alt-fire stops it from retracting its tongue, allowing you to swing around.


 * Pistol (GLOCK 17 by default, Beretta 92F/FS pistol with the High Definition pack): Has good accuracy but low damage that makes it more useful on weak targets such as headcrabs or laser trip mines. Unlike most other ranged weapons, this pistol is effective underwater. Primary fire is accurate with every shot; secondary fire is faster but less accurate. Uses the same ammunition pool as the Sub-Machine Gun.


 * Desert Eagle: This powerful pistol is the first firearm aquired by Shephard and is similar to the Colt Python from Half-Life but has less recoil and a faster rate of fire. Alt-fire activates its laser sight, which vastly increases its accuracy and power but reduces its firing rate.


 * Submachine gun / Assault rifle (H&K MP5 by default, Colt M4 Carbine/M203 assault rifle with the High Definition pack): Its fast rate of fire compensates for its poor damage and accuracy. Secondary fire launches a grenade that detonates on impact. Uses the same ammunition pool as the pistol.


 * Shotgun (Franchi SPAS-12): Does high damage at close range, but its broad fire cone makes it weak at a distance. It can be reloaded one shell at a time, but is slow to fully reload. Its secondary fire shoots two rounds simultaneously.


 * Rocket propelled grenade (RPG) launcher (ATGM-4000 RPG Launcher): Does a large amount of explosive splash damage. Secondary fire toggles a laser that guides the RPG to its target. The weapon can only hold one rocket at a time along with a maximum of five in reserve.


 * Hand grenade: Same as in Half-Life, this frag grenade explodes a few seconds after being thrown.


 * Laser trip mine: A high-explosive Claymore mine-like device that can be attached to walls or floors. It is set off either by damaging the mine or by breaking the laser tripwire it emits.


 * Satchel charge: A high-explosive that can be thrown a short distance. When the player equips a satchel and hits the fire button, the player's hand is shown, holding a remote detonator. Hitting primary fire again will make the satchel detonate. Secondary fire allows the player to place several satchels and detonate them simultaneously.


 * Snarks (Alternate look with HD pack) (alien weapon): Small, aggressive alien creatures that quickly pursue their target, pestering and biting, until finally exploding after several seconds (or if shot). If they cannot locate a hostile target, they will turn on the player that set them loose. When used in number, they are very effective at drawing enemies out from their cover, or distracting them from attacking the player.


 * Squad Automatic Weapon (SAW for short) (FN Minimi/M249): This light machine gun is very effective at cutting down groups of enemies, but empties quickly and has a long reload time. Its powerful recoil makes it difficult to aim; crouching while firing can help alleviate this.


 * M40A1 sniper rifle: A standard of first-person shooter (FPS) games, this weapon accurately delivers powerful single rounds, which hit instantly, unlike the crossbow bolts of Half-Life. Alt-fire activates the fixed-power scope.


 * Spore launcher (alien weapon): This is an organic weapon with a reptilian appearance, with two abilities. It feeds on spores and regurgutates them as dangerous warheads. Primary mode fires a glowing green alien spore (which can be picked up from scattered "spore pods") that does heavy damage to organic enemies. Its alt-fire mode launches a slower-moving "spore grenade" that bounces around for a few seconds and then explodes, causing even greater damage in its area of effect. Adrian Shephard seems to have some affection for the creature as well, stroking it fondly during one of the idle animations.


 * Shock Roach (alien weapon): Another biological weapon, these insect-like creatures are the standard armament of the alien Shock Troopers. When a Shock Trooper dies, it drops its Shock Roach, which can only survive a few more seconds before dying itself. If the player is nearby, it will "attach" itself and allow itself to be wielded. Note that if the player already has a Shock Roach, other Shock Roaches will attack. The Shock Roach fires electric bolts that inflicts moderate damage. It can fire off ten such bolts, but continuously replenishes its "ammunition." Attempting to use the Shock Roach underwater will instantly and fatally electrocute the player.† The Shock Roach's regenerating ammunition makes it very useful as a main weapon. (†Note that firing the weapon into the water from on land does not instantly kill enemies in the water. The electricity tends to follow the path of least resistance, so whenever a charge is conducted through some substance or object, the lion's share of the current travels along the particles on the surface or skin of the conductor, given the option. Thus, shooting electricity at water will result in a lot of dispersal along the surface, and not much effect on any of the creatures inside it.)


 * Displacer (Project XV11382): An experimental teleportation weapon, its primary fire launches a large, slow, green orb of energy that damages enemies it passes by and obliterates anything that it hits directly. In the single-player game, the alt-fire is used to transport Shephard to Xen as well as access certain "hidden" areas of the game. In multiplayer, the alt-fire teleports the player to a random area of the map. The Displacer's primary fire is, in effect and damage, a homage to the BFG weapon of the Doom and Quake series, taking out any enemy short of a boss with a direct hit and dealing splash damage in a visible radius.

In the single-player campaign, some of these weapons replaced their old Half-Life counterparts (Desert Eagle - Colt Python, M40A1 - Crossbow, Pipe wrench/Combat knife - Crowbar). However, in the multiplayer game the player can carry both the new Opposing Force weapons and their Half-Life variants at the same time, while cheat codes also make it possible to possess and use these weapons in single-player mode.

Trivia and notes

 * Senior Drill Instructor Dwight T. Barnes (voiced by Jon St. John) who appears in the training section, is modelled after R. Lee Ermey's Gunnery Sargeant Hartman from Stanley Kubrick's Full Metal Jacket. The same drill instructor is also heard to say "You wanna come and knock me off?" when standing on top of a rope climbing platform. This line is said by Sgt. Hulka in the film Stripes during a similar training exercise.
 * To kill the Pit Worm, Shepherd must activate the "valve" and the "gearbox," an obvious reference to developers Valve Software and Gearbox Software.
 * If the player enters Freeman's Xen portal, the game will end, accusing the player of trying to rewrite history - "Evaluation terminated: Subject attempted to create a temporal paradox."
 * The name "Foxtrot Uniform" is derived from the NATO phonetic alphabet, referring to letters "F" and "U". It is a military euphemism for "fucked up," sometimes preceded by "Alpha" (for "all", represented by "A"). See also SNAFU.
 * Downloading the Opposing Force Aliens Addon makes it possible to witness all unused alien models found in the Half-Life pack file in the Hydrofauna and Biodome areas.
 * While playing the "Boot Camp" tutorial, the player can clearly see the developers names printed on each footlocker in the barracks facility.