We watch "Oxygen" (Series 10, Episode 5) where the Doctor, Bill and Nardole take a field trip to a space station and find out that the answer to ‘Who’s there?’ is, zombies! Whilst waiting for the inevitable punchline, Nardole regrets playing hooky from vault guard duty, Bill nearly becomes one of the walking dead (can’t sue unless The Walking Dead is capitalized... oh damn it...), and the Doctor belatedly pays the price for giving away too much regenergy the prior series. But finally finds an appropriate use for the sonic sunglasses! Huzzah!