One of the season's most striking upsets punctured Northampton's growing momentum towards a long-awaited Guinness Premiership title. Crushed beyond recognition during a one-sided first half, Saints were hugely fortunate not to change ends dead and buried.
Defence was the one area of their game still intact, but the referee, J P Doyle, almost steered Northampton and their lacklustre attack back into contention with a rash of second-half penalties.
The Saints arrived having won six Premiership games in a row to face a Harlequins team still recovering from a dreadful 46-6 hammering at Gloucester, their third consecutive league defeat.
"We didn't turn up and we played badly," said the North-ampton director of rugby, Jim Mallinder. "We got outmuscled. We had penalties that could have changed the scoreline, but missed four of them.
"Quins get in your face and work at stopping you playing. We like to get off the front foot and play our game, but we were on the back foot for almost the entire first half, when our defence stood up well."
The visitors came armed with three members of the England squad – Ben Foden, Chris Ashton and Courtney Lawes – whereas Quins were without Nick Easter, because he is part of Martin Johnson's starting XV to face Ireland next week.
Harlequins took the initiative, running at Saints from the start, and were rewarded with a third-minute penalty for not releasing, scored by the fly-half Nick Evans. But tempers were up very early with a series of scraps testing referee Doyle's mettle. One flare-up turned a Quins scrum into a penalty against Juandre Kruger, scored by Evans after seven minutes.
Northampton's first venture into Harlequins territory earned a penalty on 10 minutes following a pulling down in the line-out offence, awarded against James Percival. But Stephen Myler's kick sailed yards wide.
From the restart, Harlequins returned to the visitors' 22, where only solid defence prevented the game's opening try. Northampton were fortunate not to go 9-0 down when Doyle awarded a scrum rather than a penalty with two Saints clearly offside at a ruck.
Northampton's management looked decidedly unimpressed with their scrummaging during the first quarter, but they had to be relieved with the defence as Lee Dixon got underneath what looked a certain try by George Lowe after he had shot across the line.
But there was no stopping Quins, and from the scrum the ball was worked towards the left, where Lowe drew the cover before passing out to the full-back, Mike Brown, who thundered over. A fine angled conversion by Evans saw the hot favourites 13-0 behind after 20 minutes.
Poor Myler had one of those barn-door days in front of goal and went on to miss a hat-trick of kicks before eventually breaking his duck six minutes into the second half. When Saints broke out to earn a penalty, Myler fired wide before doubling his team's scoreline with a second successful kick.
Evans had the chance to calm home nerves with his team's first kickable penalty of the half, but scuffed his 64th-minute kick and watched as it fell short.
But Shane Geraghty, on for Myler, did no better three minutes later, firing a kickable penalty across the face of the goal, and the game then descended, as so many do, into a late frenzy. But Harlequins were not to be denied.
Harlequins M Brown; D Strettle, G Lowe, J Turner-Hall, T Williams (Smith, 59); N Evans, K Dickson; C Jones, M Cairns (Booker, 68), J Andress, J Percival, L Stevenson, C Robshaw, W Skinner (capt), T Guest.
Northampton B Foden; C Ashton, J Clarke (Mayor, 63), J Downey, P Diggin; S Myler (Geraghty, 61), L Dickson; S Tonga'uiha, B Sharman (Long, 63), E Murray (Mujati 61), I Fernandez Lobbe (Easter, 56), J Kruger, C Lawes (Gray, 78), N Best, P Dowson (capt.)
Referee: JP Doyle (Ireland).