- You may be looking for the webcast.
New Series Prologue marked the "start" of the ninth series of Doctor Who, although it was not considered an episode proper. It was succeeded by the first episode of the series, The Magician's Apprentice.
Behind the scenes
This is a in-universe counterpart of the real world Doctor Who webisode Prologue that exists within the Doctor's universe. It was a visual meta-fictional cameo in The Zygon Isolation, where Petronella Osgood and her Zygon duplicate scroll through BBC iPlayer searching for something to watch.