In the early 21st century, a meteor storm hits a valley somewhere in the United States of America. Scientists and marines are sent to investigate the occurrence however they all disappear under mysterious circumstances. Eventually, a special team is sent by the authorities, only to discover the shocking truth: the meteor storm contained an alien parasite life form, which has taken control of the scientists as human hosts, turning them into murderous monsters. The soldiers must get to the source of the infestation and put an end to it.
Codename: Outbreak is a first-person 3D shooter in which players control a team of two soldiers. Recruits can be chosen among several that are available. The single-player campaign mode is composed of 14 missions. In each mission, the player controls one of the soldiers directly and gives orders to the other. Cooperative multiplayer for up to sixteen players is also available for 13 missions.
In contrast to most other games of this type, player characters carry only one weapon, which can be switched to different modes corresponding to standard FPS firearms: machine gun, sniper rifle, rocket launcher, etc., each with different types of ammo. A few missions call for a stealth approach, which is possible to achieve through the use of a silent laser gun mode. Items (e.g. medical kits) can be picked up and stored in the inventory.