Pegasi have a longtime tradition of gifting their primary feathers to those closest to them. Now that Twilight has wings of her own, she wishes to participate in the custom by giving one to her special somepony.
An unusual storm falls upon Ponyville, leading to widespread destruction. Rainbow Dash is badly injured while fighting the storm and Twilight must take care of her until the storm ends. Twidash.
Left only with a promise to keep in touch, Rainbow Dash struggles to convey her feelings, and to figure out what the words were that she couldn't say on that one last misty night at the train station before Twilight left.