2. Nothing at the End of the Lane/The Giants
Ah, 'An Unearthly Child'. Still to this day, one of the finest Doctor Who stories ever aired, and even Steven Moffat's favourite First Doctor story; well, there's a clear influence on the title of his first Who episode, isn't there? Amazingly though, that episode could have been so different. The three companions were originally Lola, Cliff and Biddy, not Susan, Ian and Barbara, and the Doctor had escaped from the year 5733 in search of the perfect past. Agents from the future would track the Doctor to stop him from causing carnage, and giant creatures would attack the four protagonists, who by the way, were shrunk to a miniature size. This is one of the few cases in which we can actually thank budget cuts; apparently, giant animals would've been too expensive to make in 1963, and instead, we have one of the finest pieces of television that Britain has ever broadcast. Maybe as a 100th anniversary special?