IDW seems particularly adept at crossovers that might not seem to make much sense, that seem impossible to do in a way that isn't pure crack-fic, and yet... they pull it all together. (There is an adult Batman/TMNT crossover, but that's not on sale: this was, and I had a coupon that would only work if I added three dollars. It was five off fifteen I believe, and I figured I'd give this a try because the first three issues were on sale and worked in combination with the coupon.)
So far, this seems to make sense. The Krangs create portals into an alternate universe - Batman's universe, and they release Batman's villains into the Turtle's world. When Harvey Dent, aka Two-Face, makes his way back to his own world, Batman catches Two-Face, only to hear his rambling about another world. (With Two-Face's duplicity and obsession with twos, I'm sure Batman thought it might be Dent's personal psychosis coming into play.)
And there are others: Joker and Harley, Poison Ivy, Clayface, The Mad Hatter. Maybe another? I'm too tired to go back and check. The turtles fight Clayface and push him back through a portal, so I suspect Batman will bring Clayface back to Arkham.
For the others, though? I suspect those will take place in the turtles' New York. So far this is fun, but the art is a little hit and miss for me. (I'm halfway through the second issue where a male character kinda looks like he has small boobs in the spandex, for example.)
It's not a killer for me, because it's mostly decent at the very least. And I ended up buying the last three issues before the sale ends tomorrow, because after one issue I'm invested enough to want to know. I'll probably want the original, adult mini-series, too, but not enough that I want it unless it's on sale. (I sampled the first couple issues. Good art technically, but the style is one that always makes me an unhappy reader. I may just see if this comes up on Hoopla or get it from the library, to be honest.)