KronoteK has been founded to use their time-travel-technology to set things right with history - whatever that means. One thing's for sure: The player was one second away of loosing his scalp to an Indian at Custer's Last Stand when KronoteK came by and snatched him. The reason: KronoteK themselves can't do anything themselves in order to keep the time-continuum save. So they need persons marked MIA. People that as far as history is concerned are only a ghost. Of course KronoteK isn't the only time-traveling-agency out there and soon after your first missions in the American Civil War and World War I, the problems suddenly get a lot bigger when well-equipped time-travelers enter the scene and mess everything up.

Darkest of Days is a first-person-shooter with the player and his tutor Dexter entering different periods of time even going as far back as the destruction of Pompey to fulfill their objective according to KronoteK's wishes like making sure that a specific person survives. During their travels they integrate themselves into the setting not only with the correct clothing but also with the weapons fit for the time-period. Only if things get desperate modern guns like a multiple-rockets-launcher or an high-tech assault rifle enter the scene to make things easier. But not everyone can be killed on sight.

Important persons from history glow either golden or blue. While the first are mission-critical and have to be saved at all costs. The latter can be taken out by either shooting them in their limps or using the so-called chasers, small high-energy-balls that only react to blue-glowing enemies and will take them out without ruining the timeline. Although the mission won't end if the player kills one or more blue-glowing enemies, the reward at the end will decrease from three to no upgrade points at all. These are needed to upgrade the two weapon-types the player can use (rifle and sidearm) in four categories including "Rate of Fire" and Accuracy".