Cobblers booked their place in the knock-out stages of the Checkatrade Trophy in breathtaking fashion as they launched a stunning second-half fightback to come back from 3-0 down and draw 3-3 with Southampton U21s at a sodden Sixfields on a crazy Tuesday night.
Northampton looked down and out when deservedly trailing 3-0 on the hour-mark, but they pulled off the most unlikely of comebacks with three goals in the final 18 minutes to stun their visitors and squeak through to the next round.
Sam Foley ignited hope of a fightback when poking home from close-range before Lewis McGugan smacked in a wonderful volley and then Ash Taylor, with the very last act of the game, nodded past a crestfallen Alex McCarthy to sensationally send the Cobblers through and leave Southampton deflated.
Those goals also meant a penalty shoot-out, somewhat pointlessly given that two teams both knew their fate, but it did allow Northampton to show off their prowess from the spot once again when Matt Grimes, Marc Richards, Chris Long and McGugan all converted to make it 13 successful spot-kicks out of 14 in this competition, which secured second spot in South Group H, behind Peterborough United, who beat Cambridge at the Abbey Stadium.
Earlier, Jake Hesketh caught the eye and his two well-taken goals put the visitors in control before Jonathan Afolabi's third on the hour-mark seemed to complete a sorry evening for the Cobblers, only for late drama to unfold.
As is the norm in this competition, there were usual wholesale changes to the Northampton team with manager Jimmy Floyd Hasselbaink making nine in all from Saturday's FA Cup tie, with only Taylor and David Cornell retaining their places.
The highlights included returns for Matt Crooks and Brendan Moloney, both back from knee injuries, and a Cobblers debut for new signing Ryan McGivern, but Billy Waters missed out through suspension having already picked up two yellow cards in the competition.
There were three academy players on the bench - striker James Gillard, midfielder Sean Whaler and defender Prudence Anaele - while first-team regulars David Buchanan, Aaron Pierre, Daniel Powell and Shuan McWilliams were all given the night off.
The tone for the first hour of the evening was set within just six minutes when Hesketh intercepted a stray pass, drove into the box and dinked a neat finish over the onrushing Cornell.
Sam Foley almost brought Town level with a low pile-driver that whistled marginally wide from 30 yards before neither Crooks nor Taylor were able to get a telling touch to Yaser Kasim's teasing corner.
The Cobblers were struggling to make much headway and they were a fortune not to fall further behind on 25 minutes when Callum Slattery shot straight at Cornell from 20 yards after finding space on the edge of the area.
Crooks so nearly hauled his side level with a brilliant individual effort, curling a left-footed shot fractionally wide of the mark having turned his man on the edge of the box, but there was precious little else for the home fans to get excited about.
Their audible groans, which were met by stares from an equally unhappy Hasselbaink, were a sign of their increasing discontent with Town's performance and it almost got worse as Cornell showed good handling to gather Alfie Jones' low shot.
There was a brief flurry of activity towards the end of the half from the home side, with Foley skewing a chance horribly wide, but still not enough to bring 'keeper McCarthy into action as Southampton deservedly kept led at half-time.
And, just like the first, the youngsters made a flying start to the second-half and strengthened their position just six minutes in when slick passing and clever movement carved the Cobblers apart., with Afolabi releasing Hesketh who clinically beat Cornell for a second time.
Southampton had been excellent and merited their lead, and they added a third goal before the hour-mark when Tyreke Johnson's low cross was left by a hesitant Taylor and Afolabi pounced, sweeping into the bottom corner with a first-time finish.
It was a long way back for the Cobblers but, aided by the introduction of Grimes, McGugan and Long, they did show a bit of fight and managed to pull one back thanks to Foley's excellent finish after Southampton failed to clear Long's cross.
The substitutions had given the hosts a new lease of life and they got themselves right back into the game with seven minutes remaining with a strike of superb quality.
Southampton only half cleared a corner and the ball dropped to McGugan who, lurking 30 yards out, showed wonderful technique to unleash a vicious left foot volley that dipped and swerved all over place, leaving McCarthy grasping at thin air.
That goal set up a frantic and gripping finale, and there was indeed a dramatic sting in the tale when, in the fifth of five added minutes, Taylor rose highest to head in Grimes' corner to miraculously send Town through.
There wasn't even time for a restart as referee Brett Huxtable blew for full-time.
Town also had the extra satisfaction of winning the subsequent shoot-out 4-2 thanks to a fine save from Cornell and Town's accuracy from the spot.
Cobblers: Cornell, Moloney, Taylor, McGivern, Smith, Kasim (McGugan 68), Foley, Bowditch, Crooks (Grimes 63), Hanley (Long 64), Richards (c). Subs not used: Coddington, Anaele, Whaler, Gillard
Southampton U21s: McCarthy, Valery, Wood, O'Driscoll, Cook, Jones (c), Hesketh (Little 75), Slattery, Johnson, Tella (Obafemi 62), Afolabi. Subs not used: Bakary, O'Connor, Rowthorn, Rose, Hale
Referee: Brett Huxtable
Southampton fans: 53