Scars of Freedom doesn't relate to the main game's story at all. It's set in the same world, but with new characters and a new time period. Rather than, as in the main game, having Death narrate the adventure, the two playable characters are their own narrators. And the story they tell borrows liberally from the likes of Frankenstein, Dr. Moreau, and other reanimation / body modding lore.
The first character is Libertee, a slave who is mortally wounded while escaping imprisonment and enhanced without her consent by a crazy doctor type. By "enhanced," developer CreativeForge means "stitched together from multiple body parts." And the second is -- you guessed it -- that crazy doctor, Doctor Gorman. His exact motivations are unclear, but he's not an archetypal villain, and he ends up working together with LIbertee against a nefarious group known as the "Masked Doctors."
Most of the game's core mechanics are intact. You'll still have a map view, and although this time it's not centered on prospecting, you'll still be collecting resources and making decisions that will impact the path you take through the game. Basic combat remains the same as well, but perhaps the most memorable thing about Hard West -- the poker-based perks system -- has been replaced. Befitting the narrative shift, instead of picking cards to choose perks, you'll be finding and buying organs or body parts that might prove useful. Examples of parts you can throw in are an "iron stomach" of a federal marshal that provides extra courage, or a bull's heart that increases your vitality. Essentially, you'll be turning Libertee into a monster in the name of improving her combat stats.
There are seven new missions in the Scars of Freedom scenario, and the degree of difficulty has been increased over the main game. It'll launch on March 3rd for $2.99 on PC, Mac and Linux, although original backers of the game on Kickstarter are entitled to a free DLC key. That's a low price for a considerable amount of content, but of course you need to own the original game to play the DLC. Talking of the original game, it'll get a big update of its own on March 3rd, which will fix some outstanding issues and also offer some new play modes.