Time is a sentient being, though not exactly in the same way humans and ponies are sentient. It has thoughts, feelings, and emotions, and sometimes… sometimes it grows curious. Sometimes, it'll watch the beings who it encompasses. Time has watched the Doctor for a long time. Time has witnessed just how much the Doctor has sacrificed to protect it. Time has seen just how much the universe has torn from the Doctor's soul, yet he pushes ever onward. When the Doctor shattered his own hearts once more to protect Donna Noble, Time has decided enough is enough.

The Time Lord has more than earned a reprieve, and Time has decided to grant it to the Doctor. During his crash landing back to Earth after he Regenerated, Time made a deal with Space. Just before it entered Earth's atmosphere, the TARDiS vanished… and the universe halted.

Fourteen universes to the left, a pony named Twilight Sparkle was just laying down in bed to sleep the stress of Nightmare Moon's defeat away. Just as she closed her eyes however, she was jolted awake by the Golden Oaks Library quaking like there was an earthquake, and the sound of something very large and moving very fast striking the ground nearby.

The Doctor has arrived in Equestria.