Understanding a little about Doctor Who is essential to understanding the joke in today's comic. Here is a quick summary of Doctor Who which will help those of you who don't watch or know about the Doctor:
"Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS (which looks like a police box). Along with a succession of companions, he faces a variety of foes while working to save civilisations, help people, and right wrongs."
The show started back in the 60's but it only worth watching at the start of David Tennant as the Doctor. My favorite Doctor is Matt Smith. Below are the trailers for the newest season to give you a taste: