Left 4 Dead 2 is the sequel to Valve Corporation's award-winning Left 4 Dead. Like the original, Left 4 Dead 2 is set during the aftermath of an apocalyptic pandemic, and focuses on four survivors fighting against hordes of the infected. The survivors must fight their way through levels, interspersed with safehouses that act as checkpoints, with the goal of reaching a rescue vehicle at the campaign's finale. The gameplay is procedurally altered by an artificial intelligence engine dubbed the "Director" that monitors the players' performance and alters the scenario to provide a dynamic challenge for the players as they progress. Several new features will be introduced, such as new types of infected, melee weapons, and a story-arc that connects each of the game's five campaigns together.