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Like Gearbox's other expansion packs Opposing Force and Blue Shift, Decay returns to the setting and timeline of the original story, but with different player characters: two female colleagues of Gordon Freeman, named Gina Cross and Colette Green. Dr. Cross is the model for the Hazard Course hologram, and can be seen at a point in Blue Shift. Dr. Green is a new character invented for Decay.
Although Gearbox completed a PC port of the game, Decay was never released "due to powers beyond [Gearbox's] control." As such most Half-Life players never got a chance to play through it. Decay is also not available through Valve's Steam service. As of September 23, 2008, Decay has been successfully ported by a fan-based team for the Half-Life engine.
- Gina Cross
- Colette Green
- The G-Man (Cameo)
- Barney Calhoun (Cameo)
- Gordon Freeman (Mentioned only)
- Black Ops
- Female Black Ops
The player is able to obtain most of the weaponry found in Half-Life excluding the Tau Cannon, the Gluon Gun, and the Crossbow. Also, in "Xen Attacks" the player gets the opportunity to play as a pair of Vortigaunt Slaves, thereby giving them the ability to fire energy bolts and hit other NPCs with their claws.
In the PC port of Decay, the player is able to obtain the Displacer from Opposing Force by entering a secret code in one of the levels.
GameplayUnlike the other narrative-based Half-Life games, which are single player only, Decay is designed for co-operative play. While it can be played by one person, swapping between the roles of Dr. Cross and Dr. Green, the game is intended to be played by two players in split-screen. Various puzzles and combat situations throughout the game require the co-operation of the two characters.
The game is also unique in the context of the Half-Life series by being the only game divided into separate missions, instead of a single unbroken narrative.
Decay has a ranking system for all of its nine missions, ranging from F (worst) to A (best), based on accuracy, number of enemies killed, and damage taken. If all nine missions are achieved with an A level, a bonus mission, Xen Attacks, can be played as a pair of Vortigaunts, R-4913 and X-8973. An Easter Egg hunt found in Domestic Violence can also unlock this mission in the PC port.
- Even though this is another Gearbox creation, none of the new weapons featured in Opposing Force are in the original release of Decay.