Half-Life: Blue Shift, developed by Gearbox Software, is a first-person shooter game and the second expansion pack for the original Half-Life. This game offers a fresh perspective on the story of Half-Life by portraying the events through the eyes of a security guard named Barney Calhoun. The game takes place in the same setting as the original game, the Black Mesa Research Facility in New Mexico. Barney's objective is to protect Dr. Rosenberg, a high-ranked scientist, amidst the chaos that ensues after the accident at the facility.

The gameplay of Half-Life: Blue Shift is similar to the original Half-Life, with the storyline progressing through a sequence of levels that consist of both combat and puzzle-solving. The game does not introduce any new enemies or weaponry, and players use the same guns to shoot the same aliens. However, the game does introduce a new mechanic of escorting a character to a specific location, which adds a new layer of challenge to the gameplay.