Half-Life: Opposing Force

Half-Life: Opposing Force (Commonly referred to as Op4 or OpFor) is a critically acclaimed expansion pack for 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 Black Mesa Research Facility on an undisclosed mission, but things go wrong as he finds himself fighting for survival against government agents and Xen aliens. The game is available on Steam, but due to an unknown bug that has yet to be patched the game freezes and is unplayable when Shephard hits the switch to open the door where the player sees Gordon Freeman about to go into Xen. However this can be bypassed with the noclip cheat.

Plot
The story of Opposing Force cleverly interweaves with the events of Half-Life, so that the player catches a glimpse of Gordon Freeman, the main character from the original game. Shephard 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 battle begins to favor the Xen aliens, 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 alien boss, 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 nuclear bomb 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

 * Boot Camp
 * Introduction
 * 1) Welcome to Black Mesa
 * 2) We Are Pulling Out
 * 3) Missing in Action
 * 4) Friendly Fire
 * 5) We Are Not Alone
 * 6) Crush Depth
 * 7) Vicarious Reality
 * 8) Pit Worm's Nest
 * 9) Foxtrot Uniform
 * 10) The Package
 * 11) Worlds Collide
 * 12) Conclusion

Canonicity
"See Gearbox Software"

Weapons

 * Pipe Wrench: The first mêlée weapon Shephard finds, the wrench is suitable for bashing crates and enemies alike. Alternate fire prepares for a more forceful swing.
 * Combat knife: Another mêlée weapon that delivers only modest damage, but is very fast and effective against headcrabs or already wounded aliens. Stabbing some enemies in the back can 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 the player to swing around.
 * 9mm Pistol: The most basic ranged weapon. Accurate and with average stopping power, the 9mm Pistol is unique in that it can be fired underwater.
 * Desert Eagle: A powerful semi-automatic pistol. Similar to Half-Life's Colt Python revolver, but has less recoil and a faster rate of fire. Alt-fire activates its laser sight, which greatly increases its accuracy and power but reduces its firing rate.
 * Submachine gun: Fully automatic with poor stopping power and accuracy, but high magazine capacity and rate of fire. Has an attached grenade launcher.
 * Shotgun: Powerful at close range, but has a slow rate of fire and long reload time.
 * Rocket propelled grenade (RPG) launcher: An extremely powerful, but must be reloaded for each shot. Alternate fire activates/deactivates a laser sight; with the laser sight active, the rockets will track the laser to its target.
 * Hand Grenade: A handheld explosive. Can bounce off of walls and detonates after five seconds.
 * Laser Tripmine: A high-explosive Claymore mine-like device that can be attached to walls. It is set off either by damaging the mine or by crossing through the laser "tripwire" emitted from it.
 * Satchel Charge: A potent explosive that can be thrown a short distance and detonated remotely.
 * Snarks (alien weapon): Aggressive and small alien creatures that quickly pursue their target, pestering and biting, until exploding after several seconds (or if shot). If they cannot locate a hostile target, they will turn on the player that set them loose.
 * M249 Squad Automatic Weapon: A light machine gun that 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 powerful and accurate sniper rifle that is very effective at long distances. Its bullets hit their targets instantly, unlike the bolts of Half-Life's Crossbow. Alt-fire activates the fixed-power scope.
 * Spore Launcher (alien weapon): A living weapon with a fish-like appearance, the Spore Launcher feeds on spores and regurgitates 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. Shephard seems to hold some affection for the creature, stroking it fondly during one of the idle animations. Shock troopers use the same spores as grenades.
 * Shock Roach (alien weapon): Another biological weapon, these insect-like creatures are the standard armament of the alien Shock Troopers. The Shock Roach fires an electric bolt that inflicts moderate damage. It can fire off ten such bolts, but continuously replenishes its "ammunition.".
 * 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. Both modes need 1-2 seconds to "charge". 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, 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. Note: The displacer uses the same spinning component as the gauss/tau gun, which can be seen when it charges a shot.

In the single-player campaign, some of these weapons replace their 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.

Allies

 * Black Mesa
 * Science Team
 * Black Mesa Security Force


 * HECU
 * Soldiers
 * Engineers
 * Medics


 * G-Man

Enemies

 * Black Ops
 * Female Black Ops
 * Male Black Ops
 * AH-64 Apache

Xen Aliens

 * Alien Controller
 * Alien Grunt
 * Barnacle
 * Bullsquid
 * Gargantua
 * Gonome
 * Headcrab
 * Headcrab Zombies
 * Houndeye
 * Ichthyosaur
 * Snark
 * Tentacle
 * Vortigaunt

Race X

 * Gene Worm
 * Pit Drone
 * Pit Worm
 * Shock Trooper
 * Shock Roach
 * Voltigore

Trivia

 * Senior Drill Instructor Dwight T. Barnes (voiced by Jon St. John) who appears in the training section, is modeled after R. Lee Ermey's Gunnery Sergeant 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.
 * Several areas of the level "Pit Worm's Nest" bear a striking resemblance to the trash compactor from the Star Wars films, particularly the pattern of red lights above their exit doors. Also, the Pit Worm itself slightly resembles the garbage squid from the same scene.
 * To kill the Pit Worm, Shepherd must activate the "valve" and the "gearbox," an obvious reference to developers Valve Software and Gearbox Software. The player also needs to activate a "steam" vent during this level.
 * 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." This means what happened in the original level was changed by the player. The same thing happens if the player somehow kills Gordon Freeman. However, "nocliping" via the console allows Freeman to be viewed face to face, revealing he possesses a short ponytail and sunglasses. In addition, doing this will play the "Threatening short" from the original game.
 * 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").
 * While playing the "Boot Camp" tutorial, the player can clearly see the developers names printed on each footlocker in the barracks facility. This seems to be a "tradition" in Half-Life because the same thing can be seen in both the original Half-Life and Blue Shift in the first level.
 * At the point where the Shock Trooper is first introduced, the security guard in the room can be heard saying "Have you seen the new IG-88?" "IG-88" is the name of the assassin droid which is one of the five bounty hunters chosen to capture Han Solo in the film Star Wars Episode V: The Empire Strikes Back.