Unknowingly to anyone, supply of drinks (or water) was poisoned with unknown substance. Anyone who drinks this drink/water is intoxicated (PCs may be more resistant). The substance causes acute paranoia and xenophobia - victims believe that everyone is plotting to kill/destroy/hurt them and take "protective measures", including killing the "enemies".
The party is hired/ordered to investigate a number of strange incidents: crazy killers, sudden suicides, people locking their houses shut and shooting at anyone getting too close. These incidents should be unrelated and seemingly random.
Every time there is a clue, that killer, suicidal person (etc) drank one specific brand of drink (e.g. Old Joe's Lemonade).
Cause of poisoning may be changed to fit campaign or be simply strange, e.g.: drink manufacturer built new installation and didn't clean it - artificial colors used in drink reacted with remains of anti-corrosive paint solvent creating highly active neurotoxin.
Copyright 1996 Michal Szokolo. Free for noncommercial use.