Mon-El in bondage! Or, at least, in a cage of Thanagaran Nth metal which he can't break out of. When a bad guy shoots him in the leg, we learn that he has a severe "lead allergy" that could kill him unless the bullet is removed. (Or, presumably, unless he's put into the Phantom Zone for a thousand years.)
Toward the end of the episode, Mon seems to be musing a romantic attachment to Kara (just like everyone else on the show, apparently). Okay, she's vivacious and sweet and intelligent and attractive...but face it Supergirl is no Shadow Lass. [How cool would it be if Lyrissa Mallor shows up? I mean, she first appeared in Invasion, and that's the basis for next week's crossover. An Mon could totally have the hots for her...]