After you've spoken to Chadley for the first time during your visit to Sector 7, the local Item Store owner will implore you help deal with a pest control issue.
Rat Problem
The rats you need to take out can be found in the north of Sector 7. From Seventh Heaven, head west until you come to some stairs on your right. Take them, and head through a pipe to enter an open space.
Polish off the Wererats, remembering that they call upon more. Make sure to pick up the chest off to the side, which contains 2 Grenades.
Head back to the Item Store, where the owner will ask you to return to the rats' nest. Do so, where you will encounter the Doomrats.
These are tougher than the standard Wererat, but are also weak to Ice — hit them hard, and they should go down without too much trouble.
Once you've polished them off, head back to the Item Store owner. He'll reward you with 5 Hi-Potions and will replenish your HP and MP. He'll also give you a nifty little discount on his wares.
Finishing this quest now opens up the On the Prowl sidequest.
