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 from losing 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 safe. So they need persons marked MIA - people who, 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 get a lot bigger when well-equipped time-travelers enter the scene and mess everything up.
Darkest of Days is a first-person-shooter. The player and his tutor Dexter enter different periods of time, even going as far back as the destruction of Pompey, to fulfill their objective according to KronoteK's wishes - such as ensuring 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 do modern guns like a multiple-rockets-launcher or an high-tech assault rifle enter the scene to make things easier. However, 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 weak spots or using the chasers, small high-energy-balls that only react to blue-glowing enemies and will take them out without ruining the timeline. Although killing one or more blue-glowing enemies won't end the mission, 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.