Somehow they turned a walk in the park into a fight to the death thanks to their own waste and the audacity of a Northampton Saints team that refused to go along with the script.
Their defense was incredible and their pack dominant, but Ross Byrne dropped seven points off the tee and they were not clinical enough in attack, leaving the door open throughout.
James Lowe's hat-trick and Byrne's five points kept their heads above water in front of a record crowd of 82,300, who greeted the final whistle with raw relief.
Given their early dominance it was hard to believe they needed an outstanding display of courage from Caelan Doris and Jack Conan to thwart a late Saints surge, but that was the reality of a strange game. In three weeks they will have to be better against Toulouse or Harlequins in London.
It had started so well and Leinster led after 11 minutes thanks to some magic from Jamison Gibson-Park.
The scrum half's kick was not handled by George Hendy and that led to a Leinster scrum.
With Robbie Henshaw, Joe McCarthy and Ryan Baird to the fore they won the winning line and Courtney Lawes gave away a cynical penalty.
Gibson-Park didn't hang around and he tapped quickly, catching a host of Northampton players as Lowe happily caught it, James Ramm stepped and scored.
Byrne converted and got his team going again by intercepting Fin Smith's pass. He didn't have the pace to score himself, so he looked for support and again the Saints coughed up a penalty.
Byrne went to the corner, they pulled the tail and Doris turned and charged for the line. The defenders turned him, so he threw the ball off the deck and Gibson-Park was on hand with another magical touch to punch the ball out to the waiting Lowe to make it 12-0.
The out-half missed the conversion, but the lead already looked insurmountable for the England side, who had made nine handling errors in the opening 24 minutes and were suffocated by the Leinster defense.
Andrew Porter forced a penalty which Byrne kicked over, then Northampton were in front of the kicker on the restart and Porter won another at the scrum.
This time Lawes thwarted a promising attack by getting over the ball and forcing a penalty, giving them possession in the opposition half.
They got off the blitz with some slick hands but James Ramm couldn't hold on to George Furbank's pass as the line beckoned and they had to settle for a penalty from Smith to get them off the goal line.
Baird brilliantly claimed the restart and Leinster looked for a final surge before the break, but the Saints survived when Josh van der Flier knocked in the tackle and they would have been relieved to be only twelve behind at the break.
Once again Leinster started the second half with flying colors and an offload from Tadhg Furlong put them ahead.
Baird picked up a loose ball and galloped into the '22. Somehow Robbie Henshaw managed to keep in touch on the right wing and as they swung out to the left, Dan Sheehan brilliantly kept it alive for Ciarán Frawley and he passed wide of Lowe to complete his hat-trick.
Northampton kept going and, after two big wins on their own line, Leinster's resistance was finally broken when Porter and Jordan Larmour tangled and conspired to give Hendy his own kick to score.
Smith reduced the margin to 10 with the conversion and after Byrne missed a handy penalty in the Hill 16 end, the Leinster defense had to intervene again as Gibson-Park's poaching relieved more pressure.
Still, the 10-point lead meant the Saints had hope and they got their chance when Gibson-Park struggled to deal with a kick from Tommy Freeman at the back and conceded a 5-yard scrum.
Northampton forced a penalty so they tapped the penalty and after a few close phases, replacement winger Tom Seabrook crossed into the corner to ensure a tight finish.
Smith converted and it was a three-point game with five to go. Leinster's attempts to play their way to the finish produced a cropper and the Saints countered brilliantly, Leinster almost clawing their way back and a combination of Doris and Jack Conan forced the turnover that saved the day.
Scorers – Leinster: J Lowe 3 tries, R Byrne pen, con
Northampton: G Hendy, T Seabrook try each, F Smith 2 cons, pen
LEINSTER – C Frawley (H Byrne 80); J Larmour (J O'Brien 73), R Henshaw, J Osborne, J Lowe; R Byrne, J Gibson-Park; A Porter (C Healy 72), D Sheehan (R Kelleher 53), T Furlong (M Ala'alatoa 61); R Molony (J Jenkins 53), J McCarthy; R Baird, J van der Flier (J Conan 53), C Doris (captain).
NORTHAMPTON SACRITIES – G Furbank; J Ramm, T Freeman, F Dingwall, G Hendy (T Seabrook 69); F Smith, A Mitchell (TJames 69); A Waller (E Iyogun 55), C Langdon (S Matavesi 58), T Davison (E Millar-Mills 58); A moon (T Mayavanua 69), A Coles; C Lawes (captain), S Graham, J Augustus.
Reference: M Raynal (France)