Rarity's wearing sequins again, and Rainbow's not happy about it. But when one of Rainbow's comments leads the rest of the Mane Six to suspect she's hiding something, they go on an epic quest for the truth, and end up uncovering one of Ponyville's biggest secrets. And, considering Pinkie is in on it and was never put under a Pinkie Promise to keep it, it's kind of a miracle it took them this long to find out.
Big thanks to OriDragon for editing!