From the introduction:
For city dwellers, rats are a source of constant trouble. They get into food stores, spread disease, and some of the larger species have been known to be quite dangerous. But some rumors tell of rats that are more than just rats – they are more intelligent than their common brethren, capable of executing clever plans, and can even talk in common with tiny, hushed voices. Most consider these rumors too ridiculous to entertain, but the truth is even more strange.
An introductory adventure for 1st level characters