Saints will be on the road in the Champions Cup quarter-finals after Racing Metro routed Jim Mallinder’s men 32-8 in the Pool 5 decider at Franklin’s Gardens.
The Premiership champions knew victory would secure a last eight date at stadium:mk, but they never came close to earning the success as they were vanquished by vibrant Racing.
A first-half yellow card for skipper Dylan Hartley in only his second game back from a three-week ban did not help the cause.
But it was defensive lapses that proved most costly as a double from wing Juan Imhoff and one apiece from Benjamin Lapeyre and Jamie Roberts secured a comprehensive success for the Top 14 giants.
Only a second-half effort from the tournament’s top scorer, George North, prevented total embarrassment as some of the Gardens faithful headed for the exits way before full-time.
It was reminiscent of previous home humblings endured by Saints in Europe’s top tier competition, with Racing adding their name to a list of victors that contained the likes of Leinster and Ulster.
But whereas defeats to those sides had cost Saints progression in recent seasons, Mallinder’s men had already qualified for the quarter-finals before a ball was kicked at the Gardens.
New midlands neighbours Wasps made sure of that, drawing with Leinster at the Ricoh Arena to ensure Saints would at least grab a best runners-up spot.
But the English champions were desperate to do more than just progress, hungrily seeking a home tie in early April.
And they made a bright start, with the pack putting the squeeze on at the scrum, earning two early penalties.
Stephen Myler missed the first and decided to go for the corner with the second, but Racing were getting some joy in the lineout, preventing Saints from building pressure.
And it was to be Racing who drew first blood as Hartley was binned for a tackle off the ball and fly-half Benjamin Dambielle lashed the ball between the posts.
Saints took their time to bring on Mike Haywood and, in the meantime, had a lineout, which No.8 Samu Manoa took.
It was far from straight and Racing had the scrum, using it to win another penalty, which Dambielle landed with aplomb.
Saints needed to conjure up some of the fighting spirit that has served them so well with a man in the bin this season.
They did so with a moment of brilliance from James Wilson, who chipped ahead, gathered and offloaded to North, helping to earn a penalty, which Myler landed.
North was soon in the action again, placing a grubber kick which Luther Burrell got to but was unable to gather, knocking on with the line in sight.
It was an enthralling contest, full of endeavour with Saints twice turning the ball over five metres from their own line.
There were hints of the dominant 2011 pack in the scrum, with the home forwards melting their opponents in one confrontation and winning another penalty at the next.
But Racing were bossing territory and possession, swarming all over the hosts, and, with the clock having already hit 40, they finally grabbed the first try of the game.
Saints switched off down their left and Imhoff was left with the simple task of gliding in and dotting down.
Scrum-half Maxime Machenaud hit the post with a poorly-struck conversion, but Racing held an 11-3 lead at the break.
It meant the French side had maintained their record of not conceding a try in the first half of any Champions Cup match this season - and Saints needed a stirring response after the break.
But they were to endure another horror moment two minutes after the restart as George Pisi’s pass fell to Imhoff, who kicked ahead and, with Myler slipping, raced onto the ball to score.
This time, Machenaud made no mistake with the conversion and Saints found themselves with a mountain to climb at 15 points down.
Injury was added to insult as Tom Wood was forced off and Racing were soon scoring again as Lapeyre swallow dived over the line after some horribly lax defending.
Machenaud’s conversion took the Top 14 team into dreamland at 25-3 up and things got even worse for Saints as Salesi Ma’afu was forced off.
Moments later, there was finally something to celebrate for the home side as Dickson sent North over in the corner, but Myler missed the conversion.
Burrell was soon beating the ground in frustration after missing a tackle on Imhoff, whose kick ahead was just too strong as it eluded him.
At the other end, Saints were attacking with renewed fervour, forcing full-back Lapeyre into a deliberate knock-on, for which he was yellow carded.
But it wasn’t to improve the fortunes of the home side as Racing ploughed on, with impressive Lions centre Roberts racking up his team’s fourth try.
Saints were stunned - and so were their supporters, who could only muster up a half-hearted ‘Mooj’ as former favourite Brian Mujati made his return to the Gardens turf.
And the final 20 minutes were merely a case of going through the motions as Saints were left to reflect on a rare, humbling home defeat.
Saints: Wilson (Elliott 69); K Pisi, G Pisi, Burrell (Stephenson 64), North; Myler, Dickson (Fotuali’i 55); Corbisiero (A Waller 55), Hartley (c) (Haywood 60), Ma’afu (Denman 48); Dickinson, Day; Wood (Dowson 44), Clark, Manoa (Harrison 64).
Racing Metro: Lapeyre; Imhoff (Chat 53), Chavancy, Roberts, Thomas; Dambielle (Goosen 70), Machenaud (Phillips 58); Ben Arous (Brugnaut 60), Szarzewski (c) (Lacombe 60), Ducalcon (Mujati 60); Charteris, van der Merwe (Kruger 60); Lauret, Le Roux, Claassen (Gerondeau 73).