Shephard, a 22-year old U.S. Marine Corporal assigned to the Hazardous Environment Combat Unit (HECU) from the fictional Santego Military Base in Arizona, is one of the personnel sent into the Black Mesa Research Facility to kill the aliens and later silence the witnesses, especially a scientist named Gordon Freeman, the player character in the original game. However, during the events subsequent to the "Resonance Cascade", Adrian finds himself separated from his unit and fighting for his life.
Unlike the other Marines, he works with the increasingly distrustful scientists and guards in order to make it out of Black Mesa alive. This is likely because he never received the orders to kill them, as his Osprey was shot down before his commander could inform the squad of their mission. Shephard and Freeman do cross paths during the game, but at that point Shephard is merely an observer to a climactic event from the original game. He never fights alongside or against Gordon Freeman. The G-Man appears to take an interest in Shephard's movements, even before the Black Mesa Incident. As early as three months prior, Shephard spots the G-Man during the boot camp training mission. Shephard deactivates the thermonuclear warhead brought in by the Black Ops, but the G-Man reactivates it and Black Mesa is destroyed. In the end, the G-Man reveals that he has successfully argued for Shephard's life, detaining him in some unknown void. The G-Man expresses a degree of respect for Shephard, offering praise for his ability to "adapt and survive against all odds" which "rather reminds [the G-Man] of [himself]."