A Plus Tard finished with a flourish to collar Kemboy in the final strides in a vintage renewal of the Savills Chase at Leopardstown.
Kemboy looked sure to win the race for a second time in three years, having helped set a strong gallop in the early stages before stablemate Melon took over the lead with a mile to run.
Melon went well clear at one stage, but David Mullins did not panic on Kemboy and when Melon stuttered into the second last, Kemboy reclaimed the lead.
It looked sure to be a Willie Mullins-trained winner as at that stage Allaho was third, but as he dropped away, A Plus Tard could be seen making ground.
Stepping up to three miles for just the second time, Henry de Bromhead's A Plus Tard (15-2) hit overdrive having jumped the last with many lengths still to find and he ended up providing jockey Darragh O'Keeffe with the biggest win of his career by half a length.
The race lost some of its lustre when the front two in the betting, Minella Indo and Delta Work, departed within two fences of each other early on the second circuit.