Prophesy says that Twilight Sparkle will not outlive her friends. To preserve the Princess, her friends must survive. They may not be immortal - but one year can become a thousand at the speed of light.
Pinkie Pie finds herself in a position where she can no longer recall the years she has spent in Ponyville, as if the memories were erased from her mind entirely. She must rediscover herself, her love for her friends, and her passion for life.