At first glance, it seems White can restrain Black’s a-pawn with Bb2+, then coast to victory by queening the b7- and c7-pawns, both teetering at the cusp of promotion. Yet the win is not served on a silver platter — Black brings just enough guile to spark a slow-burning duel.

1.Bb2+ Kh6!?

A random king move would concede a straightforward win to White, but Black is cunning from the very outset. By tucking the monarch away on h6, they quietly harbour hopes of snaring White in a perpetual. At this point, 2.b8=Q? runs into 2...a1=Q+! 3.Bxa1 Qa3+! 4.Kb7 Qf3+! 5.Ka7 Qa3+! — and White has no way to escape the checks. Nor is 2.c8=Q? any better: 2...a1=Q+! 3.Bxa1 Qa3+! 4.Kb8 Qd6+ 5.Qc7 Qf8+ 6.Ka7 Qa3+, and the checks go on — a draw. The only winning move, incredibly, is one no casual eye would suspect:

2.b8=N!!

An underpromotion! White forgoes the queen and summons a knight instead — a humble sentinel, quietly standing watch over the king.

There can be no perpetual now: after 2...b1=Q+ 3.Bxa1 Qa3+, say, White has 4.Na6.

2…Qe2!?

Black could have chosen other moves, but this is the most testing continuation — if they manage to win the bishop, a1=Q becomes a serious challenge for White to reckon with. For instance, 3.c8=Q? is careless. After 3...Qxb2, the looming threat of a1=Q leaves White with no choice but to force a draw by repeatedly checking the black king. Thus, White must relocate the bishop — but to where exactly?

3.Bh8!! — the only move that keeps the win alive.

3.Bc3? is the logical try, but it fails to 3...Qc4! 4.Bh8 Qg8! 5.Bb2 Qb3! 6.Be5 Qe6! 7.Ba1 Qe1! — the bishop finds no safe haven. After 8.Bd4 a1=Q+! 9.Bxa1 Qxa1+! 10.Kb7 Qg7, the result is a positional draw.

Likewise, 3.Bd4? Qc4! and 3.Bf6? Qe6! meet an identical fate, transposing into the same unfavourable line.

3...Qe8

4.Bc3!

Of course, 4.Bb2 Qe2, and so on, is just running in circles.

4...Qe3

5.c8=Q 1-0

The bishop drops anchor on c3, a safe harbour at last. Once again, 5.Bh8 Qe8, etc., merely wastes time, making no progress.

White commands a clear material edge, full control of the a-pawn, and a boulder of a passer poised to roll. But it took five razor-sharp moves to arrive here — full credit to Black’s resourcefulness.

Mate in 4