This was the day that Harlequins, once the fancy dans of England, refused to die. The team that finished at the head of the Aviva Premiership will make their first appearance in the final at Twickenham on 26 May, thanks to a gritty, never-say-die spirit.
Perhaps they should have lost, once Northampton had taken an eight-point lead with 14 minutes remaining. But so badly did the Midlanders manage what followed that they promptly gave away their advantage. Even when Quins took the lead for the final time, two minutes remained of a match strewn with penalties. But Ryan Lamb's restart failed to travel the necessary 10 metres, giving the home team the scrum and a chance to wind down the clock.
By that stage, the two Irishmen who have been so vital to Northampton's rise and rise in the last three years were off the field. Roger Wilson, the No 8 whose 10 minutes in the sin-bin around half-time had shown Northampton at their best, and the centre James Downey were forced off by injury. They had been the heartbeat of their club's effort.
They needed to be, because the scrum on which Northampton have set such store was rocked to its foundations. Last week was good to Harlequins in terms of England selection (nine of them will go to South Africa next month) but this game could be the making of Joe Marler, the loosehead prop. His ability in the loose has long been evident but here he needed to scrum and against a former Springbok, Brian Mujati, he did so.
Since Marler, 21, also scored on the end of a long rolling maul the only Quins try, the conversion of which made the difference, you could say he enjoyed his afternoon. For so long Quins have looked to Nick Easter and Nick Evans to be the game-breakers – the younger generation are now stepping up.
Conor O'Shea, their director of rugby, said: "Last year we lost in the league to Northampton here, we were held up over their line. The difference between the inches of last year and this year in terms of the belief of this team is just enormous and our front row – what a performance."
The nearest Harlequins have been to a Twickenham appearance was a 2009 semi-final that they lost to London Irish. Now they are a far more rounded team, even though their midfield was snuffed out by the physicality of Northampton.
Evans put his side into the lead with the first of six penalties but they did not enjoy that luxury again for more than 50 minutes.
Whatever ambitions the two clubs may have had to play an entertaining game were blown away by errors at the breakdown that allowed Evans and Lamb to exchange kick for kick; even when Maurie Fa'asavalu made two massive dents in the Northampton defence, Quins could not finish. Wilson received a yellow card five metres from his own line but Northampton rode out the storm and were still three points ahead when he returned.
Harlequins dominated territory in the third quarter but Lamb's interception allowed Northampton to break out. George Pisi then put Vasily Artemyev in the clear. Artemyev was playing only because Chris Ashton failed a late fitness test – the Russian made ground before finding the faithful Downey in support. When he was dragged down Lee Dickson was on hand to score the try, close enough in for Lamb to have converted. But the fly-half missed his target.
Typically, Chris Robshaw led his side back into the game, with the able assistance of Mike Brown. A penalty reduced Northampton's lead and another gave Harlequins the line-out 10 metres out. Easter secured possession and slowly, ever so slowly, Harlequins mauled infield, for Marler to touch down beneath a heap of bodies.
Harlequins M Brown; T Williams, G Lowe, J Turner-Hall, S Smith (M Hopper, 70); N Evans, K Dickson; J Marler, J Gray, J Johnston, O Kohn, G Robson, M Fa'asavalu (T Guest, 68), N Easter, C Robshaw (capt).
Northampton Saints B Foden; V Artemyev, G Pisi, J Downey (T May, 76), P Diggin; R Lamb, L Dickson (capt); S Tonga'uiha (A Waller, 65), A Long (R McMillan, 62), B Mujati (P Doran-Jones, 57), M Sorenson, C Day, J Craig (T Harrison, 63), R Wilson (B Nutley, 76), P Dowson.
Referee A Small (London).
Pens: Evans 6
Pens: Lamb 6