Knock Knock. Who’s there? Doctor...

Knock Knock
Who’s there?
Doctor
Doctor who?
That’s a great T.V. show, isn’t it?

This was presented as a joke on the internet, and so I am going to tell why it is funny, however, I have to also present a more likely reason for why it exists as well.

The humor in this joke has to be sarcasm because the only way that I can imagine this exchange to be funny is if the final line is delivered in a sarcastic manner. Sarcasm is insulting, and insults are funny, so the jokester here is playing this joke for laughs at the expense of people that like the T.V. show doctor who.

The other explanation for the existence of this "joke" is that the "jokester" is just using the knock-knock joke format as a clumsy segue to introduce a topic of conversation that they honestly care about and would like to talk about. This is a subversion of the joke, a non-sequitur, and perhaps an example of anti humor, but if used in this way it is certainly not a joke.