Revenge was sweet for Saints as they produced the most heroic of performances to claim a sensational 18-9 win against Leinster at the Aviva Stadium.
Jim Mallinder’s men had been mauled by the Irish side 40-7 at Franklin’s Gardens a week earlier, but they refused to suffer the same fate in Dublin.
Tries from George North and Jamie Elliott, allied with five points from the boot of Stephen Myler and a Kahn Fotuali’i drop goal made it a magical evening.
Leinster could only respond with three penalties from the boot of Ian Madigan as they were the ones left shell-shocked on this occasion.
It was a gargantuan performance from Saints, up there with the greatest they have ever produced on the European stage.
There were echoes of Belfast last year, when Northampton won 10-9 having been beaten 25-6 by Ulster at the Gardens a week earlier, but this tasted even sweeter.
Leinster had been expected to win again. To take another step towards the Heineken Cup quarter-finals. But instead Saints kept themselves in the hunt in Pool 1.
From the oldest member of the team to the youngest, Tom Collins, who came in for Dom Waldouck on the day of the game, they stood up and held firm.
Collins emphasised the belief in the side early on as he picked up the ball out wide, kicked through and raced towards the ball before being bodychecked by Rob Kearney.
After a visit to the TMO, referee Jerome Garces opted against a yellow, but Saints had a penalty and they made Leinster pay.
After some patient approach play, the ball came out to Luther Burrell, who sent Wales star and centre partner for the night North over with a clever inside pass.
Myler made the conversion and Saints were already enjoying a much better opening few minutes than in the horror show at the Gardens.
Madigan did reduce arrears with a penalty on 11 minutes, but there was no sign of Northampton being overawed on this occasion.
Instead it was Leinster making the early errors, knocking on on a number of occasions and giving possession back to Saints, who were using it well.
Ken Pisi was looking far more assured at full-back, aside from one kicking error, which wasn’t to prove costly as hooker Sean Cronin failed to gather with clear ground ahead.
Saints were standing their ground and looking threatening on the counter-attack with North a potent weapon.
His break was halted but Leinster were penalised by Garces, giving Saints the chance to crank up the heat from a lineout drive.
Lee Dickson had thought he’d scored, but was denied by the TMO before the away team again went for the corner from another penalty earned in the build-up.
But Leinster, with the most-vaunted defence in Europe, refused to crack, bringing roars of approval from the home fans as Madigan kicked the ball clear.
The Irish team were struggling to get into their stride in attack, though, looking a million miles away from the slick outfit that ran riot at the Gardens.
Consequently, Saints were growing in belief as the game went on, with only a Myler miss from the tee preventing them from going in seven points ahead at the break.
Four points was an identical lead to the one Northampton had held at Ulster a year earlier as they went on to record a memorable 10-9 win in Belfast.
And just as on that occasion the home team kicked a penalty just after the restart to bring the deficit down to one point, with Madigan doing the business for Leinster here.
But Saints again responded, turning up the pressure by kicking successive penalties to the corner.
Leinster again produced a huge defensive effort to resist a string of drives towards their line and relieved the pressure to a backdrop of cheers from the home faithful.
Luke Fitzgerald, scorer of a hat-trick at the Gardens a week earlier, did his best to get his team back on the front foot, but his sizzling break was halted as Saints won a penalty.
At the other end, Pisi’s eyes lit up as he darted through the home defence, but his final pass was just behind Elliott, who would have had a clear run across the whitewash.
Saints did get on the board just a couple of minutes later, though, as Garces gave penalised Leinster at the scrum and Myler made the kick, despite whistles from the home fans.
With 10 minutes left, the belief was seeping out of every Northampton pore as the men in green, black and gold continued to show their determination at every breakdown.
But Madigan’s penalty cut the gap to one point once more, making it 10-9 to Saints, the same score as they won by at Ravenhill a year earlier.
Replacement scrum-half Fotuali’i responded with a superb left-foot drop goal to make it 13-9 before Leinster pushed and pushed with one final late surge.
But Saints, with some massive defending, resisted for more than a minute on their own line before Jamie Heaslip knocked on, Elliott gathered and ran the length of the field to score.
Myler missed the conversion, but it didn’t matter.
Saints had produced the most heroic of performances to win in Dublin, leaving players and fans to celebrate long into the night.
Leinster: R Kearney; D Kearney (Kirchner 60), O’Driscoll, D’Arcy (Gopperth 79), Fitzgerald; Magidan, Reddan; McGrath (Bent 57), Cronin, Ross (Moore 60), Toner, McCarthy (Cullen 73), Ruddock (McLaughlin 57), Jennings, Heaslip (c).
Replacements: Dundon, Cooney.
Saints: K Pisi; Elliott, North, Burrell, Collins; Myler, L Dickson (Fotuali’i 53); A Waller (E Waller 70), Hartley (c) (Haywood 73), Ma’afu (Mercey 53), Lawes (Dowson 73), Day, Clark, Wood, Manoa (Dickinson 67).
Replacements: G Dickson, Stephenson.
Referee: Jerome Garces