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.

The story's listing on BBC iPlayer. (WC: The Zygon Isolation)

You may be looking for the real world television story.

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)

Behind the scenes

This is a version of Under the Lake that exists within the Doctor's universe.