Doctor Who: 10 Best Episodes Of The Last 10 Years

Following Blink's formula of a "Doctor-lite" episode, Turn Left from Series 4 took a trip into an alternate timeline where Donna never met the Doctor. Thanks to a fortune teller and a time beetle, Donna was given the chance to relive the moment she chose to work at H. C. Clements, the job that led her to her ex-fiancé Lance and eventually to the Doctor. The second time she headed out on her job search, she turned right instead of left and, because she never met the Doctor, she wasn't in The Runaway Bride to stop him in his destruction of the Racnoss hatchlings. Without the Doctor, Earth became more vulnerable, and the subsequent attacks he had stopped - including the Judoon hijacking of Royal Hope Hospital, the crashing of the Titanic replica - now wreaked unimaginable havoc on the human race.

What made Turn Left such a stand out episode was its creation of an alternate timeline where the Doctor and Donna had never met. Never before had we seen what would have happened if a companion had never travelled with the Doctor, and it really drove home how important the companions are to the Doctor. Also, the alternate outcomes to earlier episodes further illustrated how important the Doctor is to Earth.