The fantastic worlds of Jules Verne, H.G. Wells and Arthur Conan Doyle, in a steampunk setting where space travel and exploration is a more "civilized" thing.
The game presented an alternate history in which certain discredited Victorian scientific theories were instead found to be true and have led to the existence of new technologies. In the setting, Thomas Edison invented an "ether propeller" which could propel ships through the "luminiferous aether" (the universal medium that permeates space, based on a now outdated scientific theory), and traveled to Mars in 1870 accompanied by Scottish soldier of fortune Jack Armstrong, where they discovered that the planet was inhabited. By the time of the game's setting in 1889, the great powers have used Edison’s invention to extend their colonies and interests to the inner planets of the solar system. Venus and Mars have been colonized by The United Kingdom, Germany, France, and Russia have colonized both Venus and Mars Belgium has only colonized Mars and Italy has only colonized Venus whilst Japan and the United States maintain economic and scientific enclaves on Mars. There are no colonies or bases on the Moon. Only the United Kingdom maintains a (scientific) base on Mercury.
The inner planets reflect an evolutionary progression, the planets nearest to the sun being younger than those farther out. All planets have Extraterrestrial life, and most bear native sentient species. Mercury is primeval, tide locked and possesses only rudimentary lifeforms. Venus is a vast swamp world dominated by hulking reptiles and lizard men. The Moon is an airless dead world, but with mysteries hidden deep beneath the surface. Mars is an ancient desert planet in decline, divided into warring decadent city-states clinging to a failing system of canals. Vulcan has died and become the asteroid belt. Due to limitations in technology the outer worlds remain unreachable and unexplored. There are also hints that some worlds may have terrain hidden beneath their surface.
One of the treasures that spurred the Europeans to Mars was "liftwood": a rare cultivated plant with anti-gravity properties that allowed for the construction of giant floating ships. While the Earthers used Martian sky galleons at first, they later constructed their own armored, steam powered flyers.
Since wireless was not invented yet in 1889, communication between Earth and Mars is handled by orbital heliograph stations. The game contains much more detail on the flora, fauna, and peoples of the planets. The majority of the published material is centered on Mars.
Source: Wikipedia, "Space: 1889 - Setting", available under the CC-BY-SA License.