I totally wasn't expecting this, and what a surprise it turned out to be, good thing I saw it over on ign though, io9 spoils the surprise in the title. Geez.
THIS IS A 7-MINUTE PREQUEL MINISODE TO THE DAY OF THE DOCTOR 50TH ANNIVERSARY SPECIAL CONFIRMING ONE OF THE MOST AMAZING SECRETS IN THE DOCTOR'S LONG, LONG LIFE. WATCH IT. NOW.