I'm a well read grad student who's bluntly honest about all things, although I try to be most honest about myself.
They both have a virus and half the clues needed to figure out who gave them the virus. They are forced to work together, or admit defeat and die. And when Batman realizes that he can't just let anyone, even Joker, die, he also realizes he has to save himself to save others from Joker.
In the end, this felt a little forced, much like that last sentence in that last paragraph. It's all a little convoluted and forced. I did find the reveal of the villain to make sense: they are smart enough, and have enough force of will to pull this off. But even that felt a little disappointing, a little anticlimactic.
This graphic novel used European artists and was set in various cities in Europe, which was the main draw for me when it was on sale before Batman Day. And that was the main reason to read this for me: the art was spectacular and it was a mini-tour of Europe. It's still the main reason to read it, because the plot didn't feel like one of Batman's strongest. (Nor his weakest. It was just slight disappointment and my star rating reflects that. I certainly don't regret reading, or even buying it, but I may not reread it so much as flip through the digital pages for the art.)
If you don't mind a lackluster story, and are in it for the art, though, I highly recommend this. Otherwise, give it a go if you see it at a library, but if you're just looking for a solid Batman story, I'd skip this in favor of much better stories.