A Non-Linear Mystery Adventure
From the introduction:
This document does not give you a step-by-step way to run the module; it gives the DM basic tools to run a sandbox-style adventure. It gives a problem for the player characters to solve.
It also provides non-player characters, their backgrounds and
motivations, and ideas about how to handle certain encounters and situations that may come up in an adventure, but for the most part it does not assume that the players will follow any particular course of action.
For four to six 1st - 3rd level characters