Dirty Secrets is a large sprawling network of sewer tunnels, hidden rooms and other unsavory places which makes it easy to tie into any narrative. Take this following prompt as inspiration for your own game:
Sent to deal with problems under the city, the heroes venture down into its underbelly. The smell overtakes their senses while vagabonds scurry into the shadows. What the heroes seek only these 'creatures' know. How can trust be gained?
Dirty Secrets is constructed to be a mission within a mission. To acquire information from the sewer-folk the heroes will to have to rid the sewers of more sinister entities which lurk nearby.