As the game begins, Shephard is part of the clean-up operation. After his transport aircraft is taken down by an Alien Aircraft and he is cut off from the rest of his unit, the “clean-up” mission is abandoned and Shephard joins forces with the scattered survivors seeking escape.
Before the first chapter of Half-Life: Opposing Force, a short introduction is shown.
As Adrian Shephard and his fellow HECU comrades are approaching the LZ (Landing Zone 117) at the Black Mesa Research Facility, his plane (codenamed Goose 7) is shot down by alien aircraft, along with several other V-22 Ospreys (the first one is codenamed Goose 3).
The rest of the opening sequence consists of Shephard continually losing and regaining consciousness, seeing conflict between the Xen aliens and the Marines who survived the crash.