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Imagine a universe where square worlds spin around gemstone suns. Where planets lie cradled in the roots of an oak tree so vast its leaves twirl around brightly burning suns. Where ships of wood sail the void between worlds, and battle each other with catapult and ballista, spell and sword. Where an asteroid may be a safe harbor, a slaver's den, or a hungry creature eager to devour any that pass by. Where daring swashbucklers and scoundrels race for fantastic treasures and literally touch the stars. Where terrifying beasts with the power to destroy whole worlds roam.

Spelljammer was originally conceived as another setting for Advanced Dungeons and Dragons, but instead of making yet another stand-alone setting, Spelljammer was designed to link together all of the settings of the time into a single universe. Or perhaps multiple unified universes, because each setting has its own Ptolemaic universe, called a Sphere. All of the various Spheres float within a space called The Flow, filled entirely with a substance known as Philostigon. Philostigon is gaseous, highly flammable, rainbow-colored, cannot exist within a Sphere, and non-soluble in water. This means you can play as a character from just about any established D&D setting- though Ravenloft is set in the demiplane of dread (and is thus off-limits), and Athas (the setting of Dark Sun) has been blockaded by Gith pirates for centuries.

This is a 3.5e D&D campaign, starting at fifth level. Given the eclectic, cosmic clusterfuck that is Spelljammer, I'm allowing pretty much any WOTC-published source if you run it by me first. Just remember that something led you to accept a contract to work for an eccentric dwarven mercenary company that hops around the cosmos.