Leading scorer Scott McGleish leapt from the substitutes bench to clinch Colchester United's thrilling comeback victory and a place in the Southern Area Final of the LDV Vans Trophy and a local derby with Southend.
McGleish struck after 61, 86 and 95 minutes as Colchester hit back from a two-goal deficit. Yet McGleish started the night on the subs bench, replacing Greg Halford in the 33rd minute.
The tie swung on the 57th minute dismissal of home defender Ian Sampson for the second of two yellow cards for fouls, brandished by Kent referee Phil Crossley.
The Cobblers, who could have been forgiven for holding back five days before the visit of mighty Manchester United in the FA Cup, had appeared to be home and dry thanks to a double from Richard Walker.
The Blackpool loan man, whose three-month spell at Sixfields now expires, opened the scoring with a near-post header from Martin Smith's cross in the 14th minute and coolly added a second five minutes after the break when he controlled a Paul Trollope cross before beating Richard McKinney off a post.
But four minutes after the home side had been reduced to ten men, McGleish turned in a Wayne Andrews cross and Colchester's remarkable record of away success in this competition was set to roll again.
There were only four minutes left on the clock when McGleish timed his near-post run to perfection to head in another right-wing cross from Andrews. It was no more than the visitors deserved after a barrage of second-half pressure.
All Colchester's wins in the LDV Vans competition this season, at Cheltenham, Yeovil and Wycombe had come on their travels and the Cobblers became their fourth scalp when McGleish gleefully thumped in a back-post header from a Rowan Vine cross five minutes into the extra period.
The Cobblers had ten minutes to salvage something, but were clearly out on their feet.