a) 22...Kh8 23.Nexf7#

b) 22...gxh6 23.Qg4+ Kf8 (23...Kh8 24.Nxf7#) 24.Qh5 Re7 25.Qxh7 and mate is difficult to avoid

c) 22...Kf8 This seems to be the strongest defence. Now 23.Nhxf7 leads to a good attack (and a plus pawn for white though I don't see the mating attack in this)

I spent a long time looking at 21 Nxf7, but I finally gave up on it and starting looking for other candidate moves. I now think the key must be

21 Nh6+

This is more forcing (in that the move gives check) and of black's three legal moves, one of them (21...Kh8) clearly is not playable, due to 22 Nexf7#.

That leaves us with

- 21...Kf8 and
- 21...gxh6

to consider.

After

21...Kf8

white can simply play

22 Nhxf7

and be up a clean pawn with a much better position, or go for what's behind the curtiain with

22 Nd7+ Ke7 23 Ng8+ Kd8

but I don't see that going anywhere.

We need to also look at

21...gxh6 22 Nd7

This last move covers f6 twice (with the knight and the bishop on b2), as well as cutting of f8 as a flight square for the black king.

The threat now is 23 Qg4+ Ng7 24 Qxg7#.

I'm still unclear on how this goes down, black has lots of tries in this position. It seems to me he can give back the piece in a number of different ways and avoid mate.

Huh? 24...Nexf6 and White loses a Bishop.

I might just be pulling a blond moment here, but why did you choose 24... Kd6?

22... Kf8
23.Nd7+ Ke7
24.Qh4+

A)24...Kd6
25.Be5#

B)24...Nf6
25.Bxf6

B1)25...Nxf6
26.Qxf6+
and black has a bad position (I'd like to be white here, is all I'm saying :P)

B2)25...Ke8
26.Nxb6+

and the black queen comes off the board...

(Note that, if 25...Kd6 leads to the same mate as (A))

C)...f6

and I'm not so sure, but what comes to mind is the combination

25.Qxh6
26.Qxh7+

The reason I thought that 21.Bxe8 was necessary was that the knight on e8 can jump to d6 and protect f7 in certain lines. So I wanted to eliminate that key protection for f7...