Tutorials - One 4 Nine mod for Left 4 Dead

'One 4 Nine' is a four-level campaign for Valve Software's Left 4 Dead 1. The setting is the Nevada Desert. The U.S. Army have built a Military base around an ancient 'alien tomb', and after years of trying have finally managed to open it. The four survivors - Francis, Bill, Zoey and Louis - have heard rumors of a fortified safe zone located in the U.S Army's base, but have no knowledge of the tomb, nor what will await them upon arrival...