Chess Problem – eh?

I’m struggling to see why a chess problem has the answer it does. Does anyone see why Nf6 is the right answer here? Looks crazy to me. Surely the knight can be taken without consequence by g7 or Nf5?

Pgn is:

[Event “”]
[Site “”]
[Date “2011.08.24”]
[Round “”]
[White “Player 1”]
[Black “Player 2”]
[Result “*”]
[ECO “”]
[SetUp “1”]
[FEN “3r4/2rN1ppk/4p3/1Rp2n1n/2P2P1p/1P3P2/5B1P/3R1K2 w – -“]

1. Nf6+ * { isn’t the knight here simply undefended? }