Black Knight: g4 -> e3 (check)

White gives up (if he didn't already, because he's in a lousy position)

Possible moves for White:
White Pawn f2 -> e3, but then

Black Rook: g8 -> g3 (check)
White King: g2 -> f2 (for obvious reasons)
Black Queen: h5 -> h3

I don't think I need to explain the rest of the game.

Have a nice day.