En route to Black Mesa, Adrian Shephard’s helicopter is shot down near some cliffs. Before he can receive his orders, he is rendered unconscious. The helicopter crashes onto an electric fence. The surviving marines find themselves in a fierce battle against several Vortigaunts. The marines drag Adrian away from the crash site during their retreat, but all sustain mortal wounds.
Chapter 1: “Welcome To Black Mesa”
Adrian comes to, finding that he has been saved and cared for by a member of the science team in a makeshift hospital. The doctor tells him that a radio was left near the crash and that it could be used to call for help. Adrian arms himself, and heads for the crash site, past a security booth where the G-man is instructing a security guard to keep the door locked.
Adrian finds the radio, and is told by the marine on the other end that they have begun pulling out of Black Mesa, as there is far too much opposition. The marine tells him that he should be able to reach the evac by taking the underground rail system. Adrian begins to navigate some subterranean tunnels in that direction.
Adrian finds a section of rail, but it has been sealed by secure access fire doors. Before he can continue on, high-voltage electricity causes explosions that flood the room with radioactive waste, and the only exit is locked. The G-man unlocks the door and vanishes, leaving Adrian to narrowly escape.
Making his way through a destroyed section of the rail system and past the green goo, Adrian finds a semi-functional train.
Chapter 2: “We Are Pulling Out”
The train takes Adrian through Sector E Subsurface Transport Level A, and he arrives shortly at an area (Look at Barney transit Map) where some members of the security and science teams are hiding out. A race-x shock trooper warps in, kills a guard, and captures a scientist. Past this area, Adrian finds the remaining marines and the evac point.
A commanding officer radios Cooper, telling him that airstrikes are commencing, and that he should help pinpoint targets or find cover. He then orders Adrian to head to the helicopter in the hanger and prepare for evac. However, before Adrian can reach evac, the G-man shuts and locks the hangar door.
Chapter 3: “Missing In Action”
Traveling through a pipe maintenance area, Shephard makes his way past the ventilation system near some Admin offices and a cafeteria, into the main recirculation fans. Further vents lead through a headcrab-infested flame-testing area. Shephard blows a hole in the floor, finding more vents to travel through.
The vents lead Adrian to an underground garage complex. The main areas are sealed, but the service elevators and vehicle maintenance bay are open. A dying marine warns him of a pack of pit drones up ahead that killed his squad.
Adrian makes his way through storage areas 2 and 3, past the vehicle repair bay, and into the elevator shaft. He climbs down to the first floor, cuts the power to Elevator 2, and uses the cable to climb to the third floor. Going through a several offices on that floor, he eventually reaches a radio. Hearing the message, a marine and engineer who were also left behind climb into a maintenance area and help Adrian cut through to another elevator.
Chapter 4: “Friendly Fire”
Reaching the next floor of offices, Adrian fights past several vortigaunts and up into a conference area where the displacer gun had been designed. A collapsed section of ceiling leads up to a maintenance area.
Adrian finds a radio there, hearing that black ops have opened fire on the squad on the other end of the line. Climbing higher through the debris, he encounters a medic who tells him that there is another extraction point in Lambda sector, and that the nearby rail system should lead to it. Adrian and the medic rescue an engineer from some black ops, and then cut through the door to the Lambda sector transport system.
Teaming up with more marines, they fight past heavy black ops resistance in search of a train car. Finding one, Adrian sets off down the line. However, the track is mostly sealed. Adrian proceeds through the vents instead.
Chapter 5: “We Are Not Alone”
Past the vents, Adrian hears a scientist urging Gordon Freeman to leap into a portal to Xen. After Gordon’s jump, the room begins to destruct. Another portal appears, and Adrian jumps into it.
Appearing on Xen, Adrian jumps from asteroid to asteroid, collecting a displacer gun from a dead scientist before ending up above another portal. Small balls of light are periodically sucked into the portal. Adrian jumps in.
Chapter 6: “Crush Depth”
Adrian reappears back in Black Mesa, in Level B of the Hydrofauna Studies Laboratory, where aquatic Xen aliens are studied. Moving through an area where the alien leeches are studied, a scientist leads Adrian to a teleportation device, which takes him to an area where Icthycanthus are studied. Past some race-x, Adrian manages to enter one of the Icthy tanks, which is connected to the drainage system of the Black Mesa Research Facility Biodome Complex in Sector E.
Chapter 7: “Vicarious Reality”
Climbing out of the drainage system, Adrian heads for Specimen Observation Area 01 in the Biodome, where an experiment to see how houndeyes adapt to Earth’s environment has been ended by a Race-X attack. Killing the X, Adrian moves on to the adjacent Specimen Containment room. Beyond this is a system of elevators, which lead Adrian to the control room above Specimen Observation Area 02. There, a full-scale replica of the Xen environment is on display.
Moving on to the next Specimen Containment Room, Adrian finds three baby shock troopers in tanks. Past that, a little ball of light flies to the Data Control Operations room of the Specimen Testing Lab, where a holographic message to Walter (Bennet?) is waiting. The message points out Specimen 1176 in the next Specimen Containment room: a barnacle that was detached from its base. Beyond that point is Testing Lab BDRC-101, and the Specimen Cultivation rooms where spores and barnacles are grown.
Shephard reaches Specimen Observation Area 03, where two tentacles are being kept. The area is still under construction, and a vent is jutting out. Two more balls of light are sucked into the vent. Adrian follows them.
The vent leads to a maintenance area. The door to a service elevator is blocked with debris but marines were there, as a radio has been left behind. A marine on the other end radios for help, as his squad is under attack from a large worm creature in the waste sector. Adrian shuts off Fan Unit 1 and heads to their position.
Chapter 8: “Pit Worm’s Lair”
The vent leads Adrian to a large trash compactor in the Black Mesa Waste Processing Area 3. The door is locked, until the G-man opens it from the observation room above. Through the door, Adrian finds the squad at Basin A-03 dead. Beyond them is Waste Station 01 Toxic Disposal Basin, where the pit worm has entrenched itself. The worm is obstructing the bridge that leads out of the area; so Adrian heads down to activate the Primary Pump Gearbox in Waste Station 2 and the Main Pressure Valve in Waste Station 03.
Finding the Door to the gearbox to be locked, Adrian uses the drainage system of another trash compactor to reach the gearbox. He hits the emergency override returns to Waste Station 01.
Going to Waste Station 3, Adrian fights several pit drones and activates the Steam Vents and the Pressure Valve.
With everything activated, Adrian flushes the toxic waste, filling the Basin with radioactive waste that kills the pit worm. He activates the bridge, and continues on his way to Sewer 3 – 01
Chapter 9: “Foxtrot Uniform”
Adrian climbs out of the sewer, and finds himself on a sealed-off roadway where black ops are being trucked in and have begun to fortify the area. Making his way past them, Shephard finds a hole dug in the wall of an explosive storage area. He climbs in, and finds two marines trapped by snipers. Past the ops, Adrian runs into a large fight between the black ops and several shock troopers. Finding the only remaining path to be a sewer, he climbs in.
Something detonates the explosives, sealing Adrian in the sewer, where a large number of voltigores have come to nest. After making his way past the voltigores and their young, Adrian is helped out of the sewer by a squad of marines.
Now back in the explosive storage area, the marines tell Adrian about a standoff at the dam up ahead where their associates might need help. They fight past a large force of Race X troopers, and reach the dam.
A gargantua has been lashed down to the center of the dam and is surrounded by explosives. The marines are attempting to kill the monster with machine-gun fire, without success. In the Turbine Control booth across the water, The G-man stands talking on a cellular phone. Little balls of light are sucked into a portal behind him. The G-man disappears, and two shock troopers warp in and open fire on the garg. Adrian detonates the explosives and crawls into the large pipe exposed by the blast.
Chapter 10: “The Package”
Adrian is swept away by the current in the pipe. It takes him down to a section of desert where two marines are planning to attack the black ops. They run into a large force of black ops, and an Ordinance Storage Facility guarded by a large mortar. All access points are locked, except for one small room. Inside is a radio, where Shephard hears that some marines are under attack in the nearby bunker.
The marines are dead, but an underground path brings Adrian to where the mortar is stationed. He breaks down the fence blocking access to the Ordinance Storage Facility. He fights past several shock troopers and voltigores, and ends up in the underground parking complex beneath the facility.
Fighting past more conflict between the race-x and black ops, Adrian finds a security guard, who informs him that the black ops are planting a bomb.
Adrian stops the ops that are arming the Mark IV Thermonuclear Device, and disarms it. With a Grade C plutonium detonator, the recommended safe distance away from any explosion would be 55 kilometers.
With the bomb safely disarmed, a security guard unlocks the exit. He warns Shephard that the only path left is through the Level 4 Storage Unit, which was a staging point for the black ops before the race-x aliens arrived. On his way to the Level 4 elevator, Adrian sees the G-man hunched over the bomb, rearming it. With the gate to the bomb now locked, Adrian has no choice but to continue to Level 4.
Chapter 11: “Worlds Collide”
Adrian makes his way past the dozens of black ops in the Level 4 Storage Unit, and fights through a huge battle between the ops and race-x. More of the tiny balls of light are sucked into the huge vents around the area. Once they are all killed, a security guard opens the last door. He gives Shephard access to a stockpile of weaponry, and warns him of a huge monster up ahead that has already killed some marines. Adrian takes the nearby elevator down to largest vent system yet, and even more small lights are sucked in.
Beyond that area is a huge pink portal, and dozens of the small lights are flooding into it. Once they have all been absorbed, the gene worm materializes. Shephard fights a prolonged battle against the worm and the many shock troopers it sends in. In the end, the worm is killed, and portals begin to shoot from it. The largest portal strikes Adrian, and he is transported away.
Adrian awakes on a helicopter flying away from Black Mesa; with G-man is standing over him. Black Mesa was an embarrassment, says the G-man as the thermonuclear bomb at the base explodes in the distance. He informs Shephard that he will be detained, in order to prevent the outside world from learning about the incident. The helicopter is transported to Xen, and then into an empty area of space, where the G-man steps into a portal, leaving Shephard alone.
Opposing Force ends.