Half-Life: Blue Shift
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Blue Shift is the second mod produced for Half-Life. Much like Half-Life and Opposing Force, it takes place predominantly in the Black Mesa Research Facility and Xen. The player assumes the role of Barney Calhoun, a lowly security guard who owes Dr. Gordon Freeman a beer. However, disaster inevitably strikes, and Barney is forced to work with some scientists to escape Black Mesa.
History
Blue Shift was originally a mod developed by Gearbox Software, exclusively for the Dreamcast version of Half-Life. It also included higher detail models (High Definition Content) to accomodate the general increase in hardware power since Half-Life's release. However, for one reason or another, the game was never released on Dreamcast. The left-over content was bundled with Opposing Force and migrated to the 
PC.
Name
In keeping with the scientific motif of Half-Life, blue shift is an astronomical phenomenon that occurs when an object is moving towards the observer and the resulting energy shifts towards the blue side of the electromagnetic spectrum. The name could also refer to the fact that Black Mesa security guards wore blue uniforms, and also how Barney takes the blue tram line to work.