Pocket UFO

The game takes place within two distinct views, called the Geoscape and the Battlescape. As described by GameSpy, UFO: Enemy Unknown "...melds an SSI Gold Box RPG with a highly detailed 4X game like Master of Orion, making it in some ways two entirely different games."

The game begins on 1 January, 1999, with the player choosing a location for their first base on the Geoscape screen: a global view representation of Earth as seen from space (displaying X-COM bases and aircraft, detected UFOs, alien bases, and sites of alien activity). The player can view the X-COM bases and make changes to them, equip fighter aircraft, order supplies and personnel (soldiers, scientists and engineers), direct research efforts, schedule manufacturing of advanced equipment, sell alien artefacts on black market to raise money, and deploy X-COM aircraft to either patrol designated locations, intercept UFOs, or send X-COM ground troops on missions using transport aircraft.

The story of UFO: Enemy Unknown, set in the near-future at the time of the game's release, begins in the year 1998. The initial plot centres on increased reports of UFO sightings as tales of abductions and rumors of attacks by mysterious aliens become widespread. The nations of the world come to perceive this as a threat and attempt to form their own forces – such as Japan's Kiryu-Kai force – to deal with the crisis, but these efforts are unsuccessful. On 11 December, 1998, representatives from some of the most powerful nations in the world secretly meet in Geneva to discuss the issue. From this meeting is born the clandestine defence and research organisation Extraterrestrial Combat (X-COM), over which the player assumes control at the start of the game.

In the beginning, the player will only have access to conventional weapons, but as the game progresses, the player learns more about the enemy, their species, mutated creations, and technology. It is ultimately revealed that the "leaders" behind the alien invasion are a race known as Ethereals which possess powerful mind control abilities and enslave other races of aliens to perform their bidding and that their main base in the Solar System is located in Cydonia region of Mars. The player must then prepare the final assault team, attack Cydonia and destroy the mastermind behind the alien invasion, the biocomputer Alien Brain.

The game was originally conceived by a small British independent video game developer, Mythos Games – led by Julian Gollop – as a sequel to their 1988 science fiction tactical game Laser Squad, "but with much neater graphics using an isometric style very similar to Populous." The initial 1991 demo presented a relatively simple, two-player tactical game then known as Laser Squad 2 (or Laser Squad II), which ran on the Atari ST.

Due to bad experiences with Blade Software in selling their previous game, Lords of Chaos, the Gollop brothers (Julian and Nick) approached three other video game publishers, British Krisalis Software and Domark and the international MicroProse, eventually brokering a deal with MicroProse. Julian Gollop was especially happy about it because he greatly respected MicroProse and believed it was probably the best video game company in the world at the time.

The game was released to very positive reviews and commercial success, selling more than 600,000 units (a large number at the time) on the PC DOS platform, not counting the later ports—for the Amiga platforms and the PlayStation—and re-releases. Half of the game's net sales were in the United States, a rarity for a European title at the time. Gollop has attributed the game's North American success to its title (X-COM), as the television series The X-Files had premiered a year earlier. More than 400,000 units were sold at full price, with little marketing from its publisher.

Together with its sequel, X-COM: Terror from the Deep, its sales had passed 1 million copies by March 1997. The game earned the Gollop brothers just over £1 million in royalties. The game became very popular also in Russia, even as there were no royalties from that market as it was only distributed there via software piracy. In 1996, Computer Gaming World named it as the number one sleeper hit of all time.

The success of the game resulted in several sequels and spin-off games, as well as many unofficial remake and spiritual successor titles, both fan-made and commercial, such as UFO: Alien Invasion and UFO: Extraterrestrials. Julian Gollop himself designed the third game in the X-COM series, 1997's X-COM: Apocalypse, which was also developed together by Mythos Games and MicroProse. The game also received an unofficial sequel in the 1997 expansion set Civ II: Fantastic Worlds for MicroProse's Civilization II, in a scenario set on the Phobos moon of Mars.

Mythos Games' and Julian Gollop's own original spiritual successor project, The Dreamland Chronicles: Freedom Ridge, was cancelled in 2001 and later partially turned into UFO: Aftermath by another developer. Gollop's new X-COM spiritual successor project, Phoenix Point, was released by Snapshot Games in 2019.