UFO: Alien Invasion is a squad-based tactical strategy game in the tradition of the old X-COM PC games. However, the game is neither a sequel nor a remake of any X-COM or other commercial title. UFO: Alien Invasion has two main modes of play - Geoscape mode and Tactical mode.
In Geoscape mode, the game is about base management and strategy. You manage the activities and finances of PHALANX, controlling bases, installations, aircraft and squads of armed-response troops. You'll research new technologies and use their results in battle against the aliens. You'll launch your interceptor aircraft to shoot down UFOs and dispatch dropships in response to alien activity across the globe. You can build, buy and produce anything you like, as long as your technology level and your budget will allow it.
Geoscape mode employs easy-to-use time buttons to control the passage of time however you like, automatically pausing whenever there's an important message for you to see. Research will progress and items will be produced as time passes.
In Tactical mode, the game is about taking command of your team in various missions to combat the aliens wherever they might appear. Each soldier gets a certain number of Time Units, representing the total time they have to act during the current turn. All actions in Tactical mode require Time Units to perform. Once a soldier is out of Time Units, he or she can't do anything more until the next turn.
Your mission objectives will vary for each mission, and there are many to perform as the aliens' terrifying plot unfolds. You'll have to watch your back, be quick on your feet, and take the fight to them.
The game engine is based on a heavily modified version of ID's Quake2 Engine. This doesn't mean that UFO: Alien Invasion is a modification or even a total conversion of Quake 2. It is a stand-alone game and doesn't require Quake 2 or any other program to run. All you need to play UFO:AI is a computer running Microsoft Windows or a supported version of Linux.
The updated engine has modern OpenGL graphics and special effects, increased texture resolution, hardware-accelerated clipping of map layers for toggling between levels, a new animation system for player models, powerful artificial intelligence, and many other features.
UFO:AI also supports cooperative team play with multiple players on both sides, up to a maximum of 3 teams for each side, 6 players total.