It's a testament to the success of Half-Life that its story actually warrants retelling. As in the original, things immediately go awry, mistakes are made, people die, and your initial plan is replaced with a complicated fight for survival.
You've been sent to the Black Mesa Research Facility to locate and eliminate Gordon Freeman. Opposing Force's story recounts the events of Half-Life, but from the opposing side's perspective as implied by the title.
As such, it's appropriate that Gearbox Software's Opposing Force, the official expansion for the genre-redefining Half-Life, in turn sets a new standard of quality for future action-game mission packs.
First-person-shooter mission packs are usually mediocre: Throw together a few new levels, put in some more guns, add a spider, and you've got a commercial add-on for your aging product.
