Editing Half-Life: Decay (section)

== Gameplay ==
{{see also|Half-Life (video game)#Gameplay|l1=Gameplay of Half-Life}}
[[File:Half-Life- Decay fighting Vortigaunts.jpg|thumb|left|''Decay'' is played in a [[Split screen (computer graphics)|split screen]] mode, as seen here.]]

As is the case with the other games in the [[Half-Life (series)|''Half-Life'' series]], ''Decay'' is a [[first-person shooter]]. Like the original title it is based on, ''Decay'' requires players to engage in combat with hostile [[non-player character]]s and complete various puzzle solving tasks to advance through the game. However, ''Decay'' differs from ''Half-Life'' and its first two expansion packs, ''[[Half-Life: Opposing Force|Opposing Force]]'' and ''[[Half-Life: Blue Shift|Blue Shift]]'', in that it is designed for [[Cooperative video game|cooperative multiplayer gameplay]]. This requires players to work together to progress through the game's levels and complete puzzles as they arise in-game. Although intended to be played by two people in [[Split screen (computer graphics)|split screen]] mode, ''Decay'' can still be played by a single player.<ref name="GameSpot"/> In this case, the player can only control one character at a time, and can switch between the two characters quickly. When not in use, a character has sufficient [[Game artificial intelligence|artificial intelligence]] to defend themselves, but otherwise does not move from where they have been left by the player.<ref name="GameSpy"/>

In ''Half-Life'', players usually fight alone and only occasionally encounter friendly non-player characters who assist them, such as security guards and scientists. While ''Decay'' still features levels where this is the case, significant sections in ''Decay'' are dedicated to working with friendly non-player characters, usually escorting them to various objectives and protecting them in firefights.<ref name="GameSpot"/> An array of enemy characters from ''Half-Life'' populate the game, including alien lifeforms such as [[headcrab]]s and [[Vortigaunt]]s, as well as human soldiers sent in to contain the alien threat. The players have access to a limited selection of ''Half-Life''{{'}}s weaponry to assist them in the game.<ref name="GameSpot"/>

The game is unique in the context of the ''Half-Life'' series in that it is the only game divided into separate missions, each with a specific objective to pursue, instead of consisting of a single unbroken narrative. How players perform in each mission is ranked at the conclusion of the level as a grade from "A" to "F". This score is based on each player's accuracy with weapons, the number of kills they acquired and the amount of damage they sustained during the course of the mission. Should players successfully complete the game's nine missions with "A" grades on every level, they are given access to a bonus cooperative mission, in which they can play as a pair of Vortigaunts, as well as the ability to play through the PlayStation 2 version of the original ''Half-Life'' as a Vortigaunt.<ref>{{cite web|url=http://uk.gamespot.com/ps2/action/halflife/news.html?sid=2812076&mode=all|title=''Half-Life'' Q&A|website=[[GameSpot]]|date=September 13, 2001|access-date=October 23, 2008|url-status=dead|archive-url=https://archive.today/20120721234606/http://uk.gamespot.com/ps2/action/halflife/news.html?sid=2812076&mode=all|archive-date=July 21, 2012|df=mdy-all}}</ref>