Under the Lake (in-universe)

Under the Lake was the third episode of the ninth series of Doctor Who. It followed a The Witch's Familiar and was succeeded by Before the Flood.

It featured the Doctor and Clara and its synopsis mentioned the duo trying to give their help on an "underwater base". By 10 May 2020, the episode was available on BBC iPlayer along with the rest of series 9. (WC: The Zygon Isolation)

Important note
This is a in-universe counterpart of the real world Doctor Who television story Under the Lake 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.