Half-Life begins with a long expository sequence that situates the player in the game's environment prior to serious action or gameplay. The player-controlled character, Dr. Gordon Freeman, starts his day riding a tram deep into the heart of the Black Mesa Facility, where most of the game is set, on his way to the Anomalous Materials Lab to begin work. The tram ride gives players a perspective of the facility, setting the mood and serving as a vehicle for some comic relief moments. The ride also marks the first appearance of the G-Man, first shown as a strange man in a blue suit watching Gordon from another tram. When the player arrives at the Anomalous Materials Lab, a front desk security guard informs Gordon that a system crash occurred shortly before he arrived, which has complicated communications between the Black Mesa scientists. The player must then acquire his Hazardous Environment suit before proceeding to the test chamber.

On his way to the test chamber, Gordon witnesses caged specimens of alien lifeforms, and receives a briefing from a group of scientists. Before entering the test chamber, his co-workers inform him that his job is to push an alien sample into the scanning beam for analysis. As soon as he does so, a fictional time-space catastrophe called a "resonance cascade" occurs. This event opens a portal between Earth and a bizarre world called Xen. During this, the player is briefly teleported there and at one point sees alien life forms known as Bullsquids and Vortigaunts.