From the Cellar Games website:
Massive walking machines called "rigs" were engineered to carve into the red husk of Mars and construct new colonial structures for those who fled an Earth that didn’t want them anymore. Now war has broken out between the planet's three primary factions: the personality cult of the Shiat al-Raj'a, the Martian Autonomist Union, and the cloned zealots of "The Chosen." The working-class riggers, once responsible for building the colonies, have been called on to be the warrior class that might bring society's destruction.
MECHNOIR is an alternative Player's Guide for Technoir. It includes nine new shiny training programs, six big stompy mechs (or 'rigs'), and the two transports that bring them to the battleground. There's special rules for destroying the tags on your enemies' rigs and transports and a section for how to deal with scale when hardboiled criminals go up against hard-cased machines.
Bundled with the player's guide are three new Transmissions that outline the major colonial factions on Mars: The Chosen, the Shiat al-Raj'a, and the Martian Autonomist Union.