Retrograde analysis involves looking into a chess game’s past, rather than its future. Here’s an example from Henry Ernest Dudeney (1917):

“Strolling into one of the rooms of a London club, I noticed a position left by two players who had gone. This position is shown in the diagram. It is evident that White has checkmated Black. But how did he do it? That is the puzzle.”

The solution is unique. Can you find it?