10 Doctor Who Moments That Broke The Fourth Wall
2. The Bootstrap Paradox (Before The Flood)
Series 9's Under The Lake/Before The Flood is up there with writer Toby Whithouse's very best work on the show, just being pipped to that gold medal by Series 6's The God Complex.
It's creepy and atmospheric, with a cast of side characters who are actually likeable and well-used. It also tells one of those twisty-turny time travel stories that Doctor Who does so well, with the second episode kicking off by having the Twelfth Doctor explain the mechanics of this story directly to the audience.
More specifically, he goes on a rant about the Bootstrap Paradox, a device that Before The Flood heavily relies on. It's all very tongue-in-cheek, with the Doctor expressing his thoughts by sharing a fun story about Beethoven's 5th, before capping off his speech by playing that very same symphony on his electric guitar.
Doctor Who had used the Bootstrap Paradox many times before this story aired, so it's a little strange that Whithouse opted to include this fourth wall breaking lecture in the first place. But we're glad he did, because it's one of the best moments in the series. Only Peter Capaldi could make such complicated subject matter this entertaining!