From the introduction:
This is an adventure for Traveller, but one with a difference. Instead of playing in the far future, the players will be thrown into the Wild West. The action takes place in and around the frontier town of Bent River, though the players will discover heavy shades of science fiction soon make themselves felt. Referees should take pains at not telling the players the title of this adventure, as the presence of aliens in the Wild West should be sprung upon them. Instead, tell them it is called something like The Last Ride of McCreedy, or The Rise of the Sioux, or anything that does not have the words xenomorph or alien in it! The adventure starts off like a normal Wild West tale, but things can get weird very quickly!