Trivia: The Great Intelligence, the episode's villain, first appeared in a pair of Second Doctor serials, "The Abominable Snowmen" and "The Web of Fear". As a result, there are a few references to the two stories in the episode. First, the Doctor places pieces of the ice governess in a metal lunchbox from the '60s with a map of the London Underground on the lid. He then shows the box to the Intelligence while it is still in the snowglobe, prompting it to ask what the markings mean. The Doctor tells it, and then remarks that the Underground is a "weakness in metropolitan living" because he dislikes tunnels. In "The Web of Fear", the Intelligence and its minions set up in the Underground in the late '60s, with this episode's implication being that, as those events have yet to occur for the Intelligence, the Doctor unwittingly gave it the idea. Second, at the end, the Doctor takes another look at Dr. Simeon's business card for the Great Intelligence Institute and finds the name familiar, but cannot place it, as the Great Intelligence was not actually addressed as such in the episode. When this episode was made and aired, both "The Abominable Snowmen" and "The Web of Fear" were completely lost, with no copies in the BBC archives - a situation that changed less than a year later.