Half-Life: Opposing Force is an expansion pack for the original first-person shooter Half-Life. The game setting is the same as the one in the original game: the story takes place in the science laboratory called Black Mesa Research Facility in New Mexico at the same time. However, instead of following the original characters, players are offered to assume the role of Adrien Shephard a Hazardous Environment Combat Unit (initially antagonist organization) operator, who is sent to Black Mesa to prevent an extraterrestrial menace. Nevertheless, shooting scientists is not required, as the main threat comes out of aliens. The overall gameplay has few differences comparing to the one existing in Half-life, as far as it is based on completing the sequence of levels. Although the combat system looks almost the same, it features the non-player character’s assistance so the protagonist will receive support from scientists, security guards, and marines (soldiers, combat medics, and engineers) during firefights or while solving puzzles. Players will also face new aliens types added to the existing ones.