Just make your way through the collapsing Lambda Complex and try to survive in this catastrophe.
Lives up to the first with its great level design and puzzles.
The first 60% or so seems like a Penumbra/Half-Life hybrid which was really nice, horrifying and creative. A lovely mix of eerie atmosphere and puzzle solving. Not many mods handle it this well.
Then we finish off by getting thrown right into some action, classic Half-Life style. It's a bit difficult, though. Possibly even for those who've been playing Half-Life for years. I do wish the ending didn't come so abruptly, like the predecessor, but that doesn't take away from the mod as a whole.
I encountered some small issues throughout (like an annoying vent grate, or couple lighting bugs, etc.) however nothing game breaking.
Definitely worth a play, a proper Half-Life mod.