Set in an split timeline where an incident caused Aperture Science and Black Mesa to reverse roles; Black Mesa loses government support but becomes a highly successful and unethical private science corporation and Aperture Science becomes an underfunded government science branch suffering from its eclectic administration. The Black Mesa incident has not yet occurred in this reality, as the more capitalistic incarnation of Black Mesa have not ventured far into the world of interdimensional teleportation. Nor has the Aperture incident occurred, primarily due to a lack of government funding in AI technology. You are a corporate spy working for Aperture science, sent to sabotage the Black Mesa Upsilon facility and steal your competitors abandoned teleportation research, allowing Aperture to continue the interdimensional teleportation project...
