Can you find the surprising first move that leads to mate in 3? You must pay attention to the FEN diagram below. If you don't know how to read them, now is a good time to learn.
r3k3/1ppNp1K1/8/p7/rp1R4/4p3/8/Q1R5 w q - 0 1
It took me 45 minutes to solve it.Awful!
1.Rc6! e22.Nf6+ exf63.Re6#
1.Rc6 0-0-02.Rxc7 Kxc73.Qc1#
1.Rc6 bxc62.Qf1 0-0-03.Qa6#
1.Rc6 bxc62.Qf1 any move except 0-0-03.Qf8#
I don't have any idea of what presumably is the "mainline" on this one.
In general, mates in X don't have mainlines, usually just unique first moves, and sometimes unique combinations of a pair of moves.