After an accident, Rarity and Twilight are thrown in a race against time to stop their transformation into changelings before they become part of Chrysalis' swarm forever.
Following Twilight Sparkle's explanation of the scientific method, Diamond Tiara says the sonic rainboom is a scientific phenomenon. Scootaloo insists it is magic. The Ponyville schoolhouse is the scene for an unprecedented intellectual dispute.
A creature of old has been present in Equestria since the beginning of the rule of the alicorns, watching, waiting. He saw their fall and the sisters' rise to power. Now, for the first time in eons, he steps from watching and takes action.
Lyra and Bon Bon's adoptive son sells a drink that's insanely popular. Apple Bloom thinks he's brainwashing ponies with poison. And it's not just because he's a changeling.
A strange artifact arrives at Twilight's library and turns her into a dragon. From there, everypony she touches suffers a similar fate and the race is on to find a cure before all of Equestria falls prey!