The original version of Killing Floor, which was released as an Unreal Tournament 2004 total conversion mod in 2005 by Shatterline Productions.

Like the future retail editions of the series, it is a co-op survival horror game against waves of specimen from the Horzine corporation.. Though early versions of the mod also included a short story mode. Version 2.5 was the final version of the mod before the retail release of Killing Floor, though fans did later release up to Version 2.52.

Story Mode