Classic Who Meta: callbacks to Classic Who
Everyone knows that the Tenth Doctor episode, Human Nature was a rewrite of a Seventh Doctor book of the same name. In addition, the theme of the Doctor's resolute belief and faith in his companion(s) is crucial to both The Curse of Fenric as well as The Impossible Planet/The Satan Pit. What other Nu Who elements do you notice are callbacks to old, classic ideas from Doctor Who books, radio plays or episodes?
(Thanks to psyfi_geekgirl for this week's topic! Have an idea for a topic, too? Submit it here!)