Contingency is an upcoming co-op wave defense first-person shooter for the Source engine based in the Half-Life 2 universe. The project is currently in an open alpha state by Agent Red Productions, with its source code and developer builds all available to the public via subversion (SVN) on Google Code. An automatic updater for developer builds is also available.
As a member of an elite rebel fighting force based out of City 17, you and your four comrades have found yourself in a most hopeless situation: an unstoppable horde is approaching, one that cannot be defeated even by the likes of you and your experienced team. Facing insurmountable odds and with no hope of rescue, there's only one thing you can do: hold your ground and face endless waves of enemies in a suicidal yet honorable effort to save as many future victims’ lives as possible.
You will not survive.
The game comprised of two phases: a combat phase and an interim phase. During a combat phase, players face a specified number of enemies (NPCs) called a wave. Players have an unlimited amount of time to eradicate this wave, but if a player dies, they will not respawn. If the wave is defeated and at least one player remains, living players’ ammo is restored and dead players respawn. A relatively short window of time is then allotted for strategy refinement in preparation for the next wave. This is called an interim phase. Afterwards, another combat phase begins, spawning an even more deadly wave of enemies, and so the cycle continues with ever-increasing difficulty until all players have been eliminated.
There are currently four different types of waves: zombie waves, headcrab waves, antlion waves, and Combine waves. Waves are classified by the types of NPCs found in them. For example, during a Combine wave, players can expect Overwatch soldiers, manhacks, city scanners, etc. No two waves of the same type are exactly the same though, so while one zombie wave might feature mostly slow poison zombies, the next one could introduce a massive horde of speedy fast zombies.
Rather than providing players with a choice of fixed, predetermined classes, Contingency allows individuals to pick and choose their arsenal. Players can carry one primary weapon, one secondary weapon, one melee weapon, and one type of equipment (e.g. frag grenades) at a time. Loadouts can be changed anytime during an interim phase, but never during a combat phase.
Players' health regenerates slowly over time. Additionally, should players come across a health kit, they can pick it up and choose to heal themselves, heal a teammate, or hold onto it and save it for later. When applied, every health kit instantly restores its target's health to full.
Unlimited primary ammo is provided for all clip-based projectile weapons, but players still have to reload when their clip is empty.