A high level Wizard has confined himself inside a Prismatic Sphere spell, and, seeking total privacy, has cast Permanence upon it, seeking to hedge out the world. It acts much as a reversed Prismatic Wall, with an inside area of several acres, and the Wizard's tower at the center.
In order to escape, the PCs must find a way to cast all of the required spells, in the reverse of the normal order (beginning with Dispel Magic). The Wizard, of course, will object, and seek to stop them (Baneful Polymorph, etc.) The PCs must avoid him, find the proper spells, and bring down the seven-layered wall, one layer at a time, in order to be free.