Cowboys and Martians, presumably the Savage Worlds Mars setting.
The pitch: In the waning days of the Old West, a ruthless gunslinger called "Redface" for his ruddy-red complexion, was finally caught and due to be hanged come dawn. But in the night, he broke out of jail, killed a couple of guards, and headed off for his hideout in the Badlands. The sheriff's dead and one of his deputies formed a posse (of the PCs) to catch that varmit. You tracked him down to a cave (cursed, if you believe the natives) and followed him in.
The cave turned out to be a tunnel, but on the other end there were two moons, red sand and rock as far as the eye could see, and hey, is that a bunch o' six-armed green fellas headed your way?