During the Black Mesa incident (
Half-Life), U.S. Marine Corps Sergeant Mitchell Shephard (Mick Lauer), part of the Hazardous Environment Combat Unit (HECU), is deployed with his squad to the
Black Mesa Research Facility to cover-up the incident and kill
Gordon Freeman, a Black Mesa researcher resisting the cover-up operation. A person in an HEV suit, implied to be Freeman, ambushes and kills Shephard's squad members and severely wounds him. Shephard is recovered and rushed to a hospital in nearby
Albuquerque, New Mexico; before he loses consciousness, Shephard is approached by the
G-Man, who cryptically promises him "authority, power, and time" in exchange for killing Freeman "when the time comes". When Shephard comes to, he finds the hospital abandoned and learns Earth is being invaded by the
Combine. Shephard fights his way out of the hospital and through Albuquerque, linking up with
National Guardsman Nick (Alexander Beltman) and
Black Ops agent Adam (Vincent Cyr) along the way. In a transit station, they meet an
Army Rangers unit led by Colonel Cue (Michael Green) and learn from a radio broadcast from the
President of the United States (
Keemstar) that the world's governments have surrendered to the Combine seven hours into the invasion to avoid human extinction. The group decides to flee to
California to seek refuge on the
Pacific Ocean, per Cue's belief that the Combine lack naval capabilities. However, their train out of New Mexico is attacked by the Combine, and only Shephard, Nick, and Adam survive. In
Nevada, the trio links up with another Rangers unit led by Lieutenant Harvey (Jonathan Benlolo) and escapes with them to the
Pacific Northwest. Coming across a port, Shephard finds the container ship
Avalon Vale and its captain, Roosevelt (Oliver Colton). Nick accuses Shephard of being cursed, pointing out that both of his commanding officers, Cue and Harvey, died in the escape. Roosevelt takes command of Shephard's men, but he is killed when the Combine attack the ship. Shephard is placed in charge of the survivors and forms his own private army operating out of the
Avalon Vale. Three years later, in a Combine-administered Earth, Shephard travels to City 9 in
Alaska and discovers a Combine factory operated with
child slaves. After negotiating with the factory's administrator, Boris (Vincent Fallow), Shephard rescues the children and brings them back to his outpost to train as soldiers. Seventeen years later, the G-Man instructs Shephard to travel to
City 17 and kill Freeman. Shephard and Adam ally with the Combine under a deal with Boris, who was lifted to a high-ranking Combine position per a deal with the G-Man. Shephard and the Combine attack the Resistance base at
Black Mesa East. Shepard is captured by the Resistance before being rescued by Boris' daughter Sasha (Eli Walker); however, Adam appears and kills Sasha, urging Shephard to run. After escaping the Resistance, Shephard confronts the G-Man, who reveals that Shephard's attacker at Black Mesa was Adam, not Freeman, as part of a deal between the two. The G-Man goes on to explain that Shephard's actions have weakened the Combine, as Boris has split their forces to search for Shephard and avenge Sasha's death, thus weakening the Combine opposition Freeman faces on
his own journey, which was ultimately the intended result of the G-Man's trickery. Shephard proceeds towards a factory overseen by Boris, who orders a Combine force to kill him; Shephard withstands the assault until he is rescued by Nick and returns to the
Avalon Vale. Discovering that Adam has attempted to seize control over his army, Shephard confronts and kills him before directing the ship to set sail for a stranded
research vessel named the
Borealis. ==Development and release==