After Treviso shocked Perpignan earlier on Saturday, Northampton upset another Heineken Cup heavyweight with a nail-biting 31-27 win over Munster at Franklin's Gardens.
Chris Ashton scored two tries while Shane Geraghty contributed 21 points as the Saints showed once again that they are incredibly tough to beat at home.
Northampton now top Pool one ahead of Treviso, placing massive pressure on pre-season favourites Munster and Perpignan.
Geraghty produced a virtuoso performance in front of watching England manager Martin Johnson to help Northampton celebrate their return to the Heineken Cup after a three-season absence in a pulsating tie at Franklin's Gardens.
Northampton's fly-half scored one try and created two others which helped the Saints repeat their victory over Munster in the 2000 final in the previous meeting between these two clubs in the competition.
The three-time capped Geraghty, who has been included in Johnson's England Saxons Elite squad, turned a tight game Northampton's way with an opportunist score on the stroke of half-time - tapping and running a penalty when Munster expected him to kick for goal.
But it was Geraghty's 75th-minute penalty which finally killed off Munster's stirring fightback, led by four penalties from Ronan O'Gara and an opportunist try from Tomas O'Leary.
In the end Munster were left to regret a sleepy start in which they looked shaky early on. It was not until they were eight points down that the Irishmen began to play with any cohesion and conviction.
O'Gara was off target with an early penalty attempt, and the hesitancy of his half-back partner O'Leary was seized upon by an eager Northampton.
Geraghty nudged Northampton ahead with a ninth-minute penalty and then set up the opening try with a deft chip ahead that Munster full-back Paul Warwick fumbled. The ball bounced kindly for winger Chris Ashton, who dotted down.
Warwick partly atoned for his error when he opened Munster's account with a 50-metre drop goal, and the fightback continued with a superb try from counter-attack deep in their own half.
Keith Earls spotted that Northampton were over-committed in attack, and his kick through allowed supporting flanker David Wallace to pick up and cross for a try that O'Gara surprisingly failed to improve.
The kicking improved, Geraghty and O'Gara each slotting two penalties which left the scores level at 14-14.
Another Geraghty penalty appeared likely when Munster were penalised for handling in a ruck, but the Irish province made the elementary error of turning their backs on him. Instead of kicking for goal, Geraghty tapped the penalty and scythed through for a quick-thinking try which he also converted.
Munster probably thought they had gifted Geraghty that score. But there was no doubting the quality of the try Northampton's fly-half set up for Ashton five minutes into the second half, with a superb break and inside pass that gave the winger an easy run-in.
Geraghty took his points tally to 18 by adding the conversion but was off target with a 53rd-minute penalty - and O'Gara kept chipping away at Northampton's lead.
Munster maintained the pressure and set up a nerve-jangling finale when O'Leary snaked over from a short-side break from a scrum close to Northampton's 22. O'Gara reduced the lead to a point with a well-struck conversion.
But Geraghty had the final word, calmly slotting the winning penalty after Munster were penalised at a scrum in front of their own posts - although Northampton had to withstand a late forward barrage before they claimed the win.
Tries: Ashton 2, Geraghty
Northampton Saints: 15 Ben Foden, 14 Chris Ashton, 13 Jon Clarke, 12 James Downey, 11 Bruce Reihana, 10 Shane Geraghty, 9 Lee Dickson, 8 Roger Wilson, 7 Neil Best, 6 Phil Dowson, 5 Juandre Kruger, 4 Courtney Lawes, 3 Santiago Bonorino, 2 Dylan Hartley (c), 1 Soane Tonga'uiha.
Replacements: 16 Brett Sharman, 17 Regardt Dreyer, 18 Brian Mujati, 19 Ignacio Fernandez Lobbe, 20 Mark Easter, 21 Alan Dickens, 22 Stephen Myler, 23 Chris Mayor.
Munster: 15 Paul Warwick, 14 Doug Howlett, 13 Lifeimi Mafi, 12 Jean de Villiers, 11 Keith Earls, 10 Ronan O'Gara, 9 Tomas O'Leary, 8 Denis Leamy, 7 David Wallace, 6 Alan Quinlan, 5 Paul O'Connell (c), 4 Donncha O'Callaghan, 3 Tony Buckley, 2 Jerry Flannery, 1 Marcus Horan.
Replacements: 16 Denis Fogarty, 17 Julien Brugnaut, 18 Darragh Hurley, 19 Mick O'Driscoll, 20 Niall Ronan, 21 Donncha Ryan, 22 Peter Stringer, 23 Ian Dowling.
Referee: Christophe Berdos