Meaning of "9/11 is plain wrong" joke

My friend said to me that making fun out of 9/11 is plain wrong, and then he said it was a joke?

Honestly I don't get the joke? Why would it be plain wrong? Doesn't plain mean nothing? I'm confused.


It's a pun that only makes sense when it's spoken.

9/11 is plain wrong.

when spoken sounds like:

9/11 is plane wrong.

Plane is short for airplane. I hope I don't need to explain why airplanes are significant to 9/11...