Alien Swarm is a four player Co-Op game which is based on a Unreal Tournament 2004 mod of the same name. The plot is much like in the movie Aliens. You are part of an IAF marine squad, and your mission is to investigate with three other marines what happened in human colonies, which turned out to be an alien invasion. During the game the story is told in data pads and e-mails.
The game could be described as Left 4 Dead with a top-down perspective: the players need to reach the level end while being swarmed by aliens - preferable alive. Dead players stay dead until the level is finished. They can choose between four classes: marines with big guns, medics with healing abilities, officers which can improve the other player's skills and technicians.