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
interdimensional portal to the borderworld of
Xen, allowing the alien creatures of Xen to attack the facility. The player controls 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 Corps 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. However, Shephard is prevented from reaching the extraction point by the enigmatic
G-Man, resulting in the evacuation craft leaving 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 encounters more stranded Marine units in the wreckage of Black Mesa, and attempts to reach an unknown exit route, encountering heavy resistance from Race X and 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 neutralizing the black ops unit guarding the device, Shephard disarms it and proceeds to a nearby storage facility to attempt another escape. As Shephard departs, the G-Man rearms the nuclear device. The storage facility has become a battleground between Race X and the black operation units. 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 enough to force it back through the wormhole, but immediately 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. ==Development==