Mate in 3

As is often the case, examining the play that follows when White simply passes their turn reveals much to the solver. Note that, if Black were on move, the following variations are already prepared in the diagram:

• 1...fxe3 2.Qe2 b2 3.Qa6#

• 1...fxg3 2.Qg2 b2 3.Qa8#

• 1...b2 2.Qc2, then either 3.Qa4# or 3.Qxb1#

This suggests that the key must be a waiting move — one that also provides for 1...f3 without disturbing the set play above. Instinct points to the wK: move it off the f2–a7 diagonal, so 1...f3 can be dispatched with 2.e4 b2 3.Qa7#. The question, though, remains: where exactly should it be placed?

1.Kc6? and 1.Kb7? unfortunately interfere with g2–a8, allowing 1…fxg3! to wreck the plan. Nor does 1.Kb5? fare any better — blocking e2–a6, it is duly refuted by 1…fxe3!. That leaves only 1.Kc7! — indeed, the key move. Black stands in zugzwang: 1…f3 runs into 2.e4! b2 3.Qa7#, as intended, while every other reply is answered by the continuations outlined at the outset, reproduced below for completeness:

• 1...fxe3 2.Qe2! b2 3.Qa6#

• 1...fxg3 2.Qg2! b2 3.Qa8#

• 1...b2 2.Qc2! ~ 3.Qa4# & 3.Qxb1#

  2…Ka2 3.Qa4#

A fine block problem in Meredith form — elegant, economical, and with a key that is anything but obvious.