Progress through the game's world is continuous; although the game is divided up into chapters, the only significant pauses are when the game needs to load the next part of an environment. Opposing Force also features an extended multiplayer, incorporating the various new environments and weapons into the original deathmatch mode used in Half-Life. After release, a new capture the flag mode with additional levels, items and powerups, was created by Gearbox for the game.
For the most part the player battles through the single-player game alone, but is occasionally assisted by friendly non-player characters. Security guards and scientists will occasionally help the player in reaching new areas and convey relevant plot information. However, Opposing Force also features fellow U.S. Marines who will assist the player in combat to a far greater degree than security guards. Three types of Marines are featured in the game: the soldier will simply provide fire support for the player with a submachine gun, shotgun or machine gun, the combat medic is capable of healing the player and other non-player characters, while the engineer can cut through doors and remove obstacles, allowing the player and their squad to proceed unhindered. A selection of enemies from Half-Life populate the game, including alien creatures such as headcrabs and Vortigaunts.
A variety of new alien non-player characters, labelled "Race X", appear in the game as well, often engaging in combat with the aliens from Xen. The player also encounters human opponents in the form of a detachment of black operations units who have been sent to destroy the base in the wake of the failure of the U.S. Marines to eliminate the alien threat. A limited selection of Half-Life's weaponry is allocated to the player to defend themselves with, although several new weapons such as a sniper rifle, combat knife and a variety of alien weaponry are also present in the game.
Synopsis
Setting
Opposing Force is set in the same location and timeframe as that of Half-Life, taking place at a remote New Mexico laboratory called the Black Mesa Research Facility. In Half-Life, the player takes on the role of Gordon Freeman, a scientist involved in an accident that opens an inter-dimensional portal to the borderworld of Xen, allowing the alien creatures of Xen to attack the facility. The player guides Freeman in an attempt to escape the facility and close the portal, ultimately traveling to Xen to do so. Opposing Force, however, shows the events of Half-Life from the perspective of a different protagonist. The player assumes the role of Adrian Shephard, a U.S. Marine Corporal assigned to the Hazardous Environment Combat Unit, a specialized unit sent in to Black Mesa by the government to quell the alien threat and silence all witnesses. However, after Shephard becomes isolated from his fellow Marines, he must ally with the Black Mesa personnel and attempt to escape the base.
Plot
Opposing Force opens with Shephard riding on a V-22 Osprey with his squad. His squad discuss their deployment, somewhat irritated that they have not been told what they are being deployed for. However, as they are nearing their Landing Zone at Black Mesa, airborne alien creatures attack the Osprey formation, causing Shephard's Osprey to crash land. Shephard regains consciousness in a Black Mesa medical bay, tended to by the science team, learning that the Marines are being steadily beaten by the alien forces from Xen and orders have come through to pull out. Consequently, Shephard departs to reach an extraction point and escape the facility. However, Shephard is prevented from reaching the extraction point by the enigmatic G-Man, forcing the evacuation craft to leave without him.
Other Marines who have also been left behind team up with Shephard and attempt to reach another extraction point near Black Mesa's Lambda Complex, but en route they come under attack from black operations units seeking to thoroughly contain the situation and eliminate all survivors. Shephard makes it to the Lambda Complex alive, and briefly sees Gordon Freeman as the latter teleports to Xen in the final stages of Half-Life. To escape the teleportation chamber, Shephard is forced to enter a separate portal, briefly taking him to Xen before depositing him in an entirely different area of the facility. The facility is now heavily damaged, and it soon becomes clear that a new alien race, Race X, has exploited the situation to mount a localized invasion, attacking both human and Xen forces in Black Mesa indiscriminately. Fighting between the black operations units and Race X quickly intensifies.
Shephard soon encounters more stranded Marine units in the wreckage of the base, and tries to break through the black operations units to reach an escape point past the facility's storage unit, but meets heavy resistance from both Race X and the black operations units. A surviving Black Mesa security guard reveals to Shephard that the black operators intend to detonate a tactical nuclear weapon in the base, thereby totally sealing it off and killing everything in it. After successfully neutralizing the guarding black operations unit, Shephard disarms the device and proceeds to the nearby storage areas, intent on escape. However, the G-Man rearms the device as Shephard departs. The storage unit has become a thick battleground between Race X and the black operation units, and although Shephard manages to evade them, he is informed by another security guard that something very large is coming through an alien portal blocking the exit path.
At the portal, Shephard discovers a gene worm, a massive creature facilitating the Race X invasion. Shephard is able to wound the creature and force it back through the wormhole, but just afterwards he is teleported onto an Osprey by the G-Man. As the G-Man congratulates Shephard on his accomplishments, the nuclear device detonates in the background, destroying Black Mesa. The game closes with the G-Man detaining Shephard someplace where he can tell no one of what he has seen and cannot be harmed, pending further evaluation.
