Long Run spoiled the party as he scuppered Kauto Star's bid for a record fifth triumph in the William Hill King George VI Chase at Kempton.
It was evident Paul Nicholls' star was feeling the heat three fences from home, and a terrible mistake at the second-last from the 4-7 favourite assured the Nicky Henderson-trained runner a memorable success.
He came home well clear under amateur partner Sam Waley-Cohen from stablemate Riverside Theatre, with Kauto Star and Tony McCoy only third.
Paddy Brennan set the early pace aboard Nacarat but McCoy was keen to take close order and settled in second.
Waley-Cohen soon moved up to join them on the 9-2 chance and the trio dominated proceedings, with all three really reaching for the 14th fence as the pace turned up a notch.
The distress signals were out for Kauto Star by then, however, and turning for home it was evident the defending champion was about to be deposed.
As Long Run kicked on, Kauto Star got two out all wrong and ploughed through the top, with McCoy doing extremely well to stay in the saddle.
That sealed it for Long Run and while Riverside Theatre finished to good effect for Barry Geraghty, he was still 12 lengths adrift at the line.
It was then a further seven lengths back to a tired-looking Kauto Star.
Waley-Cohen said: ‘It's absolutely amazing. We ploughed through the first, but we have done a lot of work on the inside school to teach him how to get in a rhythm.
‘He really picked up and showed real courage as he was very long at some of them. He's a phenomenal horse and I'm so pleased he's shown people what he is capable of.
‘It's a massive honour to be on a horse like this. Kauto Star is a true champion and Tony McCoy is an idol for everyone in the weighing room - to be involved in a race like this is such a privilege.
‘I'd like to thank everyone who has helped me.’
Henderson added: ‘Sam has done brilliantly, he has worked very hard and thought about it. He got the horse into the rhythm we needed and gave him a beautiful ride.’
With his son having done the steering, owner Robert Waley-Cohen was a proud father afterwards as he said: ‘What could be better than that? There is no bigger adrenalin rush in the world.
‘That's the perfect riposte (to criticism of the jockey). He has justified my faith in him.
‘It's absolutely fantastic, what could be better than winning a Grade One race?’
Nicholls offered no excuse for Kauto Star's eclipse and admitted he never felt the horse was going well enough.
He said: ‘To be honest, I didn't ever think he was going to win during the race and there are no excuses. He just laboured a little bit and he could have jumped better.
‘Sometimes when you watch him, you know he's going to win but I never thought that today. I was never really happy at any stage. The mistake cost him second but he has done incredibly well to even come here five times.
‘He just looked like he had lost half a yard around here at the age of 11.
‘He's come back safe and sound, that's the main thing.
‘I'll sit down and have a talk with Clive (Smith, owner) but if he is sound and well there is no reason why he can't run in this year's Gold Cup. Cheltenham will suit him better than Kempton does now, he's just that bit slower.
‘Long Run was always going to be a good horse if he put it all together and he did that today.’
Smith added: ‘I didn't quite see it like Paul, there was a moment coming round the home bend when I thought all was well but suddenly he seemed to stop.’
Looking to the future for the winner, Henderson admitted a Gold Cup bid would now be tempting.
He said: ‘We've always said that around here would be a more favourable course for him than Cheltenham but looking at that, I think there's a good chance we'll be going there in March.
‘The main thing is that he stayed the three miles well and my initial reaction would be that he wouldn't want to be going back in trip for the Ryanair.
‘I was a bit worried a couple of weeks ago as to whether he was at the same level as Boxing Day but every time I asked my assistant, Tom Symonds, who rides him every day, he seemed very positive.
‘We've always known he has a huge amount of talent and I'm so pleased he has shown it today.’