Twilight knew Spike would start to grow up one day, but that doesn't make it any easier when it happens. She's sure Pinkie's right: he just needs some space, and he'll be just fine by himself while she spends some time with Celestia.
But the Princess has her own reasons for calling Twilight back to Canterlot. Two travelers in the Everfree have woken dark forces beneath the Earth, forces of fire and steel...