Mirrors. They serve to show one´s reflection, but sometimes they are used as gateways to another worlds. After discovering one such mirror and it´s creator´s notes, describing the cold and dark lives of ponies living on the other side, the Princesses decided, that now, when their Equestria is safe, it´s their duty to go and help them.
Twilight is the one to enter and quickly finds herself surrounded by everything they taught her to be evil. If just things were that simple. She is in the middle of everlasting war and those with the greatest darkness might become her new comrades.
Can she give these ponies life of happiness and friendship, just like they have it in Equestria?