1.Bg8+ Rxg8 2.Qf7+ Rg7 3.Qxg7#
Diagramed is the position after 23...Nxd4 from a game between Zukertort and an amateur player, John Adair. The full game is given below, where you can see that Zukertort missed the mate in three and resigned shortly after. Zukertort later denied that he ever had a mate available, claiming that the game's moves were misreported--see clippings below. Note that this was 1884, when Zukertort had a reasonable claim to the title of world champion.
Johannes Hermann Zukertort – John Dunlap Adair
Chicago, January 10, 1884
1.e4 e5 2.Nc3 Nc6 3.f4 exf4 4.Nf3 g5 5.h4 g4 6.g5 h6 7.Nxf7 Kxf7 8.d4 d5 9.exd5 Nce7 10.Bc4 Kg7 11.O-O f3 12.gxf3 g3 13.Bf4 Nf5 14.Be5+ Nf6 15.Ne4 Be7 16.Qd2 Rf8 17.Qf4 Kh7 18.Bd3 Nxd5 19.Qg4 Nde3 20.Qh5 Bxh4 21.Rfe1 Nd5 22.Re2 Nb4 23.Bc4 Nxd4 24.Rd2 Nxf3+ 25.Qxf3 Rxf3 26.Rxd8 Bxd8 and White resigns.
Below are some clippings from the Central New Jersey Herald regarding the scandal.
all taken from Chess Notes