With Left 4 Dead 2 on the horizon, Valve is sticking to its promise of supporting the original L4D with content. Due out this September, an all-new campaign entitled “Crash Course” is coming to both the PC and Xbox 360.

Bridging the gap plot-wise between No Mercy and Death Toll, Crash Course explains how the survivors found themselves stranded in a remote wilderness riddled with infected. Promised are new locations, dialogue, and an “explosive finale.” It’ll also be available in Versus and Survival Mode right from the get-go, so you won’t be waiting for the complete package.

On PC, Crash Course will be distributed for free via Steam while Xbox 360 owners will have to pony up 560 MS points. You can presumably thank Microsoft’s mandatory third-party pricing policies for that.

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