Dark Sector is primarily a cover-based shooter, which leads to the obvious
Gears of War comparisons. Players take the role of a government agent (Hayden) sent to the fictional city of Lasria to assassinate a rogue agent, named Mezner, who is in possession of a biological virus called Technocyte. When infected by the virus, subjects undergo a variety of mutations, causing them to transform into mindless killing machines. As these things usually go, Hayden is infected by the virus but is able to control it, giving him a weapon to use against Mezner.
Similar to Gears, gameplay is mostly based around running between areas of cover while periodically popping out to take a few shots at enemies. Levels are linear, though they are designed to give you enough wiggle room to deal with situations however you want. Although you'll mostly face small groups of enemies, you'll also come across giant enemy nests where you'll have to take out small armies. These are the areas where the game is weakest; the cover in these situations isn't the best and some enemies spawn in the worst possible places and come in such high numbers and erratic intervals that there's little sense of progression.
Multiplayer is a bit of a throwaway. Although there are two game types, Infected and Epidemic, the only one that ever seems to get any play time is Infection - and even then it is hard to find a good game. Most of the time, I was lucky enough to stumble into a game that was filled with Achievement whores who were more interested in inflating their Gamer Scores than having a competitive match. In the rare instances where I found a legit group, the gameplay wasn't compelling enough to keep the group intact for very long. During Infected games, one player is infected while the others play as soldiers and work as a team to take him down. The power balance is skewed towards the infected player; even with four guns trained on him, the only way to kill the infected player is to hit him from behind with a melee attack, but getting close to the infected player is hard because he can become invisible and cut your to shreds with his blade.
