Meisterwerke

As both parties have mate in one, the question becomes: Whose turn it is to move?. The unfortunate location of the wBa3 and pawn formation around it tells us that the white captures have been: a2xQb3 and d2xBc3 accounting for all missing black pieces. The black captures must have been e7xRf6 and two pawn captures, one after a promotion of e-pawn.

Concerning the promotions, following must have happened:

• On the h-file the white h-pawn was simply captured, after which the black h-pawn was promoted to a white-squared bishop still on the board.
• The white e-pawn has been promoted (after e7xRf6) to rook, and is still on the board, on c3.
• The white rook from a1 got captured on the a-line by the black b-pawn.
• After white's a2xQb3 the doubled black a-pawns have promoted to knights still on the board.

The task then is to unpromote the two black knights, retract the resulting pawns beyond a2 and then retract the white move a2xQb3, after which the position releases without much trouble. However, there is a further problem: It is not possible to directly take both "free" black knights (Nb1 and Nb6) to the unpromotion square a1, as they need to go through b3 square, needing a white tempo manouvre each time. But there readily exists only one such manouvre : Ng3 Re4 Ne2.. Fortunately there is an indirect way to accomplish this:

• Transfer bNb6 to e5.
• Replace wNg4 with bNe5.
• Transfer bNb1 to a1 and unpromote. Tempo can now be done with "free" wN
• Replace bNg2 with wNe3
• Transfer the liberated bNg2 to a1 using the "built-in" tempo manouvre.

Now suppose black has the move. Then the retro-play is:

-1. Rb3-c3 Na8-b6 -2. Rc3-b3 Nc7-a8 -3. Rb3-c3 Ne6-c7 -4. Rc3-b3 Nf8-e6 -5. Rb3-c3 Ng6-f8 -6. Rc3-b3 Ne5-g6 -7. Ne3-g4 Ng4-e5 -8. Nf1-e3 Nd2-b1 -9. Ne3-f1 Nb3-d2 -10. Nf1-e3 Na1-b3 -11. Ne3-f1 a2-a1=N -12. Rb3-c3 Nf4-g2 -13. Ng2-e3 Ne6-f4 -14. Rc3-b3 Nc5-e6 -15. Rb3-c3 c7-c6 -16. Rc3-b3 Nb3-c5 -17. Ng3-e4 Re4-e2 -18. Ne2-g3 Na1-b3 -19. b3-b4 and black is retro-pat!

So white has the move. The retro-play is:

-1. ... Na8-b6 -2. Rb3-c3 Nc7-a8 -3. Rc3-b3 Ne6-c7 -4. Rb3-c3 Nf8-e6 -5. Rc3-b3 Ng6-f8 -6. Rb3-c3 Ne5-g6 -7. Ne3-g4 Ng4-e5 -8. Rc3-b3 Nd2-b1 -9. Nf1-e3 Nb3-d2 -10. Ne3-f1 Na1-b3 -11. Nf1-e3 a2-a1=N -12. Ne3-f1 Nf4-g2 -13. Ng2-e3 Ne6-f4 -14. Rb3-c3 Nc5-e6 -15. Rc3-b3 Nb3-c5 -16. Ng3-e4 Re4-e2 -17. Ne2-g3 Na1-b3 -18. b3-b4 c7-c6 -19. Bd6-a3 a3-a2 -20. Bf4-d6 a2-a1=N -21. Bc1-f4 a4-a3 -22. Bd2-c1 a3-a2 -23. Bc1-d2 b4xRa3 -24. Ra1-a3 b5-b4 -25. a2xQb3 Qa3-b3 -26. Rb3-c3 b6-b5 -27. Rb5-b3 Qb4-a3 -28. Re5-a5 Qa5-b4 -29. c3-c4 Qa8-a5 -30. d2xBc3 Bb4-c3 -31. Re8-e5 Bf8-b4 -32. e7-e8=R Qd8-a8 -33. e6-e7 e7xRf6 and the whole position is finally unlocked.

So, it must be white to move and win with 1. Rxd3#

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Solution by mr.mip