Well he hasn't done any prep work or mock debates so...
I'm sure he can get away with a lot of generic, vague answers ("Syria is a mess, Obama and Hillary aren't leading. When I'm president we're going to lead again, and win again. We have to fix that mess.") there will also be some very specific questions asked, He's going to look unprepared and stupid.
I guess the question is whether the media discusses it. I feel confident saying he's going to put on the worse debate performance (in terms of having a general understanding of issues) in the modern era. You can't convince me he's going to do better than Kerry, Obama, McCain, Romney, Carter, etc etc on basic issues. And yet I wouldn't be stunned if the responses are: "defiant...strong...didn't back down...held his own, on his own terms...etc."
I want to say he bombs, the media admits it, polls show he lost bad, and Trump responds by refusing to attend the second debate due to the system being rigged.