Saints secured one of their most memorable European victories as they crushed Ulster’s 13-match winning run at Ravenhill.
A first-half try from No.8 Gerrit-Jan van Velze and five points from the boot of the returning Stephen Myler sent the travelling faithful into fantasyland with a 10-9 win.
Saints had beaten all the odds after losing 25-6 at Franklin’s Gardens just eight days earlier to the RaboDirect PRO 12 leaders.
Ulster could not string together the play to get past an incredible Saints defensive effort, which restricted the hosts to three Paddy Jackson penalties.
And Ruan Pienaar’s missed kick with two minutes to go was to prove the most costly of misses as Northampton breathed new life into their Heineken Cup campaign.
They are now just five points behind Ulster at the top of pool four, with games against Castres and Glasgow to come.
Last week, Saints were the ones shut out, this time it was Ulster, and Jim Mallinder’s men had done what they have failed to do in recent years: beat a member of Europe’s elite on the road.
Northampton were superb in the first half, answering the calls to stand up to the Ulstermen after last week’s flaying at the Gardens.
Boss Mallinder had made six changes to the side which was systematically dismantled by the Irishmen at the Gardens and his selection was paying off.
One of those decisions was enforced as hooker Mike Haywood came in for suspended skipper Dylan Hartley, and the 20-year-old’s nerves were to prove of a steely nature.
Saints got into their stride quickly, ensuring the home side were stuck in their own half during the opening exchanges.
Mallinder’s men had the bit between their teeth, fired up by the humbling at the Gardens , and some slick build-up play merited a penalty, which Myler slotted.
Saints clearly hadn’t read the script, or if they had they were ripping it up, as great hands from Ken Pisi and Soane Tonga’uiha released van Velze to score his first try for the club since joining from Blue Bulls during the summer.
Myler made it 10-0 with the conversion and the visitors were in dreamland, but the Ulster fans’ fury at referee Nigel Owens was building to a crescendo.
The Welsh official wasn’t moved by the deafening boos, but Ulster nearly had some assistance from a different source as Myler’s chip was charged down by Nick Williams.
But the big No.8 couldn’t find the pace to break clear of the Saints backline, who got back with haste to stop a score. Such defensive desire had become a theme of the away side’s play in the first period.
Owens eventually awarded Ulster two penalties, yielding sarcastic applause from the home faithful, the second of which Paddy Jackson notched to bring his side back to within seven points.
But the fly-half failed to add to his tally moments later, sliding the ball wide of the right post after Tonga’uiha had been penalised at a scrum.
Ulster had the scent of points in their nostrils and their desire to close down Samu Manoa paid off as the second row forward was suffocated in the tackle and conceded a penalty.
This time, Jackson did register his second three-pointer of the night and the wind was firmly at Ulster backs as the half-time whistle sounded.
Saints’ inability to catch kick-offs has haunted them over the years, as it did in the Heineken Cup final loss to Leinster in 2011, and the failure resurfaced as Ulster were immediately handed the second-half impetus.
They made Northampton pay, with Jackson kicking another penalty from a similar position to his second, and Ulster had roared back to within a point.
But this was a different Saints side to the one which lost to the Celtic League side last week and gargantuan lock Manoa was agonisingly held up as he dived for the line.
Tom May was to endure similar agony moments later as he knocked on from a clever Myler grubber kick
The Saints wolf was at the Ulster door, but the men from Northampton just couldn’t find the necessary puff to blow it down.
Brian Mujati was brought on to add some braun and with his input, the Saints scrum was moving forward at real speed, similar to the Halcyon days of the 2010/11 season.
But Ulster weren’t to be beaten and a brilliant break from Ruan Pienaar, flying forward and releasing to Tommy Bowe, who was superbly stopped by Jamie Elliott.
Bowe picked up what looked like a serious injury in the process and his pain will have been accentuated by what happened next as Ulster were awarded a penalty to win the game, but Pienaar missed it.
Saints, as they had done all night, ground away as the clock ticked down and a chorus of ‘Oh When The Saints’ rang around Ravenhill.
And the travelling faithful were left to toast a monumental victory as Myler booted the ball out of play and ended Ulster’s 100 per cent record.
Ulster: Payne; Bowe, Cave, Wallace, Trimble (Gilroy 56); Jackson (P Marshall 59), Pienaar; Court, Best, Afoa; Henderson, Tuohy (Lutton 79); Williams, Henry, Wilson.
Replacements: Brady, Black, McComb, Diack, L Marshall.
Saints: Foden; K Pisi, G Pisi, May, Elliott; Myler, Dickson; Tonga’uiha (A Waller 61), Haywood (McMillan 78), Doran-Jones (Mujati 53); Manoa, Clark (Sorenson 61); Wood, Dowson, van Velze (Lawes 25).
Replacements: Roberts, Lamb, Burrell.
Referee: Nigel Owens (WRU)