It's Scale's first year at the Neutral Training Grounds between the Solar City-States and the Lunar Republic. After beginning her studies in the musical arts, a piece she has written is selected to be performed at the 500th Peace Celebration. At the same time that Scale is celebrating with her friends, ponies from both sides of the border begin falling sick from an unknown illness. Each country blames the other for the deaths that begin to pile up. With all the anger in the air, harmony becomes unbalanced and the cycle of day and night stops once again. The Celebrations are canceled. Scale and her newfound friends take it upon themselves to find the real cause of the sickness and put the Celebration back on track.