Rainbow Dash discovers her element of harmony to be missing, and so asks Twilight for some help. As luck would have it, the element appears to somehow be in the world on the other side of the mirror portal. With explicit instructions to stay as far away from Canterlot High School as she can, Rainbow Dash is sent through the mirror to retrieve her element, but staying away from CHS might prove harder than she thinks....
Wait until RD finds out she won't have any wings...