Overturn is played through the eyes of Ivan Grigorovitch. Five hours after the resonance cascade, you are at the Lambda Reactor Complex, the scientists want to start an teleport sequence to get Freeman to neutralize Nihilanth, but they don't have enough crystals to start a large scale teleport sequence.

This is where Ivan comes in... He was part of the earliest research experiments on Xen creatures, so he seemed to be fit for the job. They had started the teleport sequence to an abandoned vessel that belonged to the Black Mesa research facility 50 years ago, but unknown causes had made it disappear... It reappeared recently after the cascade had happened. It had a large number of crystals from the earliest Xen research. They prepared you for the job, equipped with the new prototype HEV suit, Glock 18, and a melee weapon, they sent you off.

But what they did not know, was that this was not nearly enough equipment to live through the horrors of Overturn.
ModDB Page: http://www.moddb.com/mods/overturn
That's it guys!

PS: I'm needing modelers and coders, if anybody is interested, PM me! :D