Helpmates

I won't post any more Gargantua mate-in-one puzzles, as they're so simple to solve. I'll continue to post Gargantua mate-in-two puzzles, which of course are slightly harder to solve. I'll also try my hand at constructing Gargantua helpmate puzzles, starting with this post. helpmate-in-one puzzles are slightly harder than mate-in-one puzzles, since the solution requires an initial move by Black before White can move, which is precisely where the help comes in :-)

Puzzle of the day: below is a Gargantua helpmate-in-one puzzle; what is Black's helping move, and White's checkmate reply?

update: here's the solution