Twilight Sparkle leads Pinkie Pie, Applejack, Rainbow Dash, Fluttershy and Spike on a quest to save Rarity from the evil diamond dogs. What starts as another harmless romp quickly turns into a gritty struggle as it becomes evident the dogs will risk life and limb to resist Equestrian interventionism. Succumbing to exhausted desperation and rage, the ponies stoop to the lowest of lows and commit a deed none of them will forget.

Tensions run high as the surviving ponies cope with the fresh memories of their suffering and crimes in different ways, climaxing when the ponies finally meet to share their feelings with one another.

Also, this has ABSOLUTELY NOTHING WHATSOEVER to do with Heart of Darkness. That was the original (scrapped) concept, and the title just stuck because it's awesome and it fits.