Twilight borrows an idea from Time Turner to 'bring the past forward to you', creating a spell that in theory should allow one to see the past without taking part in or altering it. Due to Twilight's genius alterations made with too little sleep, it backfires and results in semi-actual time travel where each pony is sent back in time into their old foal bodies. Various shenanigans ensue with a chapter for each pony.

Cover art by mlplary6 on Deviantart and used with permission. See the animated version there.