Late tries from Julien Touxagas and Justin Murphy saw Catalans snatch victory over Harlequins and cement second spot in the engage Super League.
The fourth game of Millennium Magic appeared to be drifting towards a tame conclusion after Quins had opened up a 16-6 lead in the second half.
Yet the French suddenly burst into life after Touxagas picked up a loose ball and sprinted 90 metres to score with 10 minutes remaining.
Murphy then took a Thomas Bosc kick with two minutes left on the clock to cut Leeds' lead at the top to five points and extend the club's unbeaten run to nine games.
Victory had seemed to belong to Harlequins after the Londoners had hit back from Jason Croker's early try with efforts from Michael Worrincy, the influential Scott Hill and Chris Melling.
After 206 points were scored in the Cardiff event's first three matches yesterday, the majority of the contest had been quiet by comparison.
With the roof closed and noise echoing around the huge arena, the atmosphere was a strange one as the crowd built up ahead of two more intense derbies later in the day.
Croker, Super League's oldest player at 35, opened the scoring for the Dragons with five minutes gone after Harlequins had been penalised for incorrectly playing the ball.
Bosc added the extras but the Londoners produced a quick response with Hill putting Worrincy over.
Rob Purdham converted and Danny Orr went close to putting Quins ahead when he was just beaten to his own kick.
Harlequins edged themselves 10-6 ahead just before the break when Hill crashed over from a Chad Randall pass.
An Orr kick created another opportunity for Harlequins but Murphy managed to get back to beat Tony Clubb to the ball.
Harlequins stepped up the pressure early in the second half with a swift move which saw Jon Wells put Hill clear to set up Melling for his first try of the season.
Purdham, who had missed a penalty attempt with time expired in the first half, made no mistake to extend Quins' lead to 10 points.
The game appeared all but over when, out of the blue, Orr dropped the ball on the Dragons' 10-metre line and Touxagas gathered and broke away to score.
It was only his second Super League try in three years and, with Bosc converting, the Dragons were suddenly revived.
They applied more pressure and Murphy, scoring for the fourth consecutive game, snatched the game from Quins' grasp when he took Bosc's kick in his stride to touch down.
The French still needed Bosc's conversion but he made no mistake.