Half-Life Episode Two begins exactly where Half-Life ended, after accepting the job agreement with Gman Gordon freeman is sent back to the Black Mesa Research Facility, why? Its up to you to find out.
A technology demo showing the water and particle effects possible with the Raven engine, an updated Half-Life 1 engine being used by the mod.
As you can see in the video, realistic rain and water effects are possible in this engine.
This demo was run in a Core 2 Duo 3 Ghz, 2 GB ram DDR2 800 and a GTX 260 video card, you may notice that the framerates are low, that happens because there's too many ripples and rain particles in the scene being processed by the CPU, if this effect is actually used in the mod it wont have as much particle density, though the particle engine is still not 100% optimized, the final release will have performance improvements. You may also notice that the audio starts to not match the video footage at some point, I don't know why that happens.