I saw this on Saturday afternoon and was pleasantly surprised at (a) how well it was made and (b) how funny it was. Or maybe I was just in the mood for a luagh, I dunno.
The basic thing is: the turtles are still underground, Vern is taking all the credit for beating Shredder, April is on assignment, a cop named Casey Jones (a cop? Yes.) becomes an urban vigilante who can't be heard behind his mask and an NBA player slips on a slice of pizza.
Oh yea, two idiots named Bebop and Rocksteady get turned into animals by Tyler Perry and Shredder: at one point, Bebop lets go a thunderous fart which rattled the speakers, the glass in my spectacles and most of the people around me.
Oh yes, and Krang makes an appearance too.