From the introduction:
As fall begins, All Hallow’s Eve approaches. Though once a yearly reminder that the dead linger just beyond the living world, Halloween has now become a time of fun and frivolity. Even in Freedom City, where genuine monsters sometimes stalk the streets and sewers, Halloween is a time for candy, parties, fun costumes, and laughter. Protected by their spandex-clad heroes, many Freedonians feel downright invulnerable. Too invulnerable.
When a Halloween prank ends up calling down far more trouble than anyone expected, Freedom City needs heroes to protect it once again.
Monster Mash-Up is a short adventure for a group of three to five PL 10 heroes. The adventure is set in Freedom City, but can easily be transplanted anywhere in the world that celebrates Halloween and has a large, old theater to gather an audience into.