From Introduction:
Diesel Mice is a game built on the FirstFable system by OneBookShelf, inc. Just like FirstFable, Diesel Mice is released under the Creative Commons Attribution Noncommercial Share Alike license. As such you are freely allowed to use, add to, and release your own modifications to this book.
Diesel Mice is designed to be simple enough that it can be used to introduce 8 to 12 year olds to the activity of roleplaying. Diesel Mice is designed with the intention of taking the FirstFable framework and building a setting reminiscent of many animated films (specifically, those dealing with mice living in a world just below the notice of the human world).
The rest of this book explains how to customize characters and how the game system works. It also provide advice for running the game, keeping the players engaged, and making sure everyone has fun. Finally, it finishes off with Raid On The Steel Mill, an introductory adventure. To run a game of Diesel Mice, you’ll need this book and a handful of six-sided dice (you can get these at many “big box” stores for very little money, or you can buy cool-looking colored dice from game stores — the love of multicolored dice starts early!)
For those of you familiar with FirstFable, there are a few changes from the original rules. The biggest change is that Diesel Mice does not make use of Character Books. There are character types included in this book in the Mouse Types chapter. Also, Special Things and Weaknesses have been reworked a little bit. The rules-as-written in FirstFable made you use Stars from Special Things and then optionally add a dice roll to them. Instead of that, in Diesel Mice you roll the dice, and then decide if you want to add Stars from the Special Thing. Also, Special Things provide Permission (a concept that has been discussed in the Fate community) meaning that Special Things may not mechanically influence a roll, but still give the ability to make the roll when a character without that Special Thing would not be given even a
chance. Using a Weakness recharges a Star on a Special Thing. Finally, there is an appendix on Advanced FirstFable system that is designed to be used as a next step for roleplayers who find FirstFable a little too simple.