Half-Life: Sum takes place during and after the resonance cascade in which you play as the tough as nails HECU grunt Jane Doe as she has to wade through 11 sprawling maps. A compilation of the previous released mods "HTC", "Autonomy Lost" and "Abeyance".
Unfortunately no one ever seemed to notice that this area was a almost complete rip-off of an area from the unreleased mod Half-Life: Nightwatch.
See a comparison here: Moddb.com
Of course this was not done for the sake of malice but rather a little nod to the original developers since we never got to play it.