Planet (The Caretaker)

The Twelfth Doctor and Clara Oswald were once held prisoner on a desert planet. While chained to a post, the Doctor told Clara to pass him the vibro-cutters so they could break free, only to be informed by her that she had left them in the pocket of her other jacket which she had left at home. The Doctor assured Clara that the sand piranhas would get them long before they starved to death. (TV: The Caretaker)

Behind the scenes

 * It is never explained how the Doctor and Clara came to be held prisoner on the planet nor how they eventually escaped.