Worlds closer to the galatic core than Known Space are --or were-- home to intelligent speciers.  Some learned of the core explosion in time to flee.  Destroyer of Worlds opens in 2670, ten years after Juggler of Worlds closes; with refugee species fleeing in an armada of ramscoops in the direction of the  Fleet of Worlds.  The onrushing aliens are recognized as a threat; they have left in their trail a host of desolated worlds: some raided for supplies, some attacked to eliminate competition, and some for pure xenophobia. Only the Puppeteers might have the resources to confront this threat--but the Puppeteers are philosophical cowards... they don't confront anyone.  They need sepoys to investigate the situation and take action for them.
