An item has been stolen from a temple/mage/lord etc, the thieves trailed to a portal leading to an unknown plane/realm. The PC's are hired to follow and retrieve the item and/or scout the realm. The realm escaped to is from the Gamma World game. Several thousand years after an atomic war, patches of technology still exist. Most survivors are animal and/or human mutants and have a mix of equipment. Laser pistols, bow and arrows, smart missiles, swords, armalite rifles, battle axes, war robots etc. Survivors are TOUGH and many have physical and/or mental mutations, as the weak ones have already died out. Several technological installations still exist, guarded by robots etc. PCs must trace the item, find the current owners, retrieve the item and return before radiation traces in the atmosphere slowly kill them. (Optionally, the portal is now set so that it can only be used by someone carrying the stolen item, hence stopping the PC's escaping or more raiders coming through). Equipment bought back may or may not work. PCs with laser pistols, rocket launchers and mini-tanks are worrying in fantasy worlds.