Plot: Half-Life: Blue Shift is an expansion pack for the first-person shooter video game Half-Life. It was developed by Gearbox Software with Valve and published by Sierra On-Line. Blue Shift was the second expansion for Half-Life, originally intended as part of a Dreamcast port of Half-Life. Wikipedia
Plot: Half-Life is a series of first-person shooter games developed and published by Valve. The games combine shooting combat, puzzles, and storytelling. The original Half-Life, Valve's first product, was released in 1998 for Windows to critical and commercial success. Wikipedia
Plot: Half-Life 2 is a 2004 first-person shooter game developed and published by Valve. Like the original Half-Life, it combines shooting, puzzles, and storytelling, and adds features such as vehicles and physics-based gameplay. Wikipedia