1: Lost

During a flash flood, Donatello is lost saving Mikey, and though no body is found, most of them fear the worst...

Meanwhile Donatello wakes up injured in an underground tunnel system far from home, and he doesn't remember anything but his name and that he needs to get to Manhattan, New York.

A mutated bat named Fisher promises to help him get back to Manhattan, in exchange for him defending her from another mutant with his martial arts training he doesn't remember learning.

And so begins a trip unlike anything else Donatello remembers...

(Set in the 2012 Nickelodeon series universe.)

I dunno who owns TMNT, but it's not me. Fisher is my OC, though.