From the introduction:
Ah, yes. The town has a problem with rats that are larger than usual coming out of the sewers, attacking people, and causing damage to shops. Any brave soul who is willing to go into the sewers to take the rats out will be grouped together with other honest adventurers and given a key to the iron grates that bar entry to the city sewers.
This adventure is designed to be run with just the basic rules that can be downloaded from the official D&D website.
A 1 to 2-hour adventure for 1st level characters