The Dragons became the third Welsh side to bomb out of the Challenge Cup this weekend as they let a commanding lead slip from their grasp in an 11-try thriller.
With 17 minutes to go at Rodney Parade, they were leading 39-24 and seemingly on course for the quarter-finals.
But they leaked three tries in the closing stages, amid a yellow card for Wales hooker Elliot Dee, with Northampton replacement back Tom Collins claiming the winning score four minutes from time.
The Saints will now be at home to either Harlequins or Ulster in the last eight, while the Dragons will be left to ponder on what might have been.
They follow Cardiff Blues and the Ospreys in going out of the second-tier competition at the last 16 stage.
That leaves the Scarlets as Wales’ only remaining hope in Europe, with Glenn Delaney’s team at home to Sale in the Champions Cup on Sunday.
Yet it had all looked so promising for the Dragons for much of the match on their return to their Rodney Parade home.
They had won three of their previous four matches and that confidence and self-belief was there for all to see during an impressive first-half performance.
After their dominant scrum enabled them to open the scoring through the boot of Sam Davies, they scored their first try on 12 minutes.
Off a lineout steal, they put together a sustained attacking passage, with Jamie Roberts involved twice.
The veteran centre made a big dent out on the right and then, a couple of phases later, he threaded through a perfect grubber for Ashton Hewitt to run on to and touch down.
There was then a double boost for the home side following a TMO review.
Matthew Screech’s try from a close range ruck was awarded after a lengthy review and there was also a yellow card for Northampton wing Ollie Sleightholme for pulling back off the ball in the build-up.
It was Screech’s fourth touchdown in his last three games, which is some haul for a second row.
On 34 minutes, there was a third try from the Men of Gwent and a second for the predatory Hewitt.
A lovely fingertip flick pass from Sam Davies saw the ball reach the wing out on the left and he held off the two Saints half-backs to produce an excellent finish.
Unfortunately, Hewitt suffered a leg injury in scoring and had to leave the field.
The visitors gave themselves some hope three minutes before the break when Wallaby wing Taqele Naiyaravoro spotted a gap and scampered over for a converted try that made it 22-10 at the interval.
Within a minute of the restart, Northampton were right back in it as wing Sleightholme finished strongly out on the right after Naiyaravoro broke from deep and scrum-half Alex Mitchell put in a cross kick.
But they then suffered a second double whammy as they were reduced to 13 men with the sin-binning of two forwards from the same passage of play.
Lock Alex Coles was yellow carded for going offside and blindside flanker JJ Tonks followed him to the bin after a video review confirmed a late hit on Sam Davies.
The Dragons capitalised on their two man advantage, with fly-half Davies stepping inside to the line, only for the Saints to defy the numbers by striking themselves through Mitchell.
Going into the final quarter, the action swung from one end to the other, with replacement prop Leon Brown powering over for the hosts and Saints skipper Teimana Harrison responding by going over out wide.
Then came the dramatic finale with Dee yellow carded for offending near his line and Naiyaravoro crossing for his second ahead of Collins’ late winner.
What a game and what a wretched weekend for the Welsh regions so far.
Dragons: A Hewitt (2), M Screech, S Davies, L Brown; cons – S Davies (4); pens – S Davies (2).
Northampton: Tries – T Naiyaravoro (2), O Sleightholme, A Mitchell, A Harrison, T Collins; cons – J Grayson (5); pen – J Grayson.
Dragons: J Williams; J Holmes, A Owen, J Roberts, A Hewitt; S Davies, R Williams (capt); G Bateman, E Dee, L Fairbrother, J Davies, M Screech, A Wainwright, H Keddie, R Moriarty.
Replacements: T Davies, J Reynolds, L Brown, B Carter, D Baker, G Bertranou, E Lloyd, N Tompkins.
Northampton: Mallinder; Sleightholme, Dingwall, Hutchinson, Naiyaravoro; Grayson, Mitchell; Auterac, Haywood, Painter, Moon, Coles, Tonks, Adendorff, Harrison (capt).
Replacements: Fish, Waller, Heffernan, Bean, Matavesi, Taylor, Tuala, Collins.
Referee : Alexandre Ruiz (France)
Kick-off : 5.30pm