The Steelbacks were left stunned on Vitality Blast opening night as Leicestershire Foxes produced an incredible chase to win by four wickets at the County Ground.
Northants had looked to be in total control as they posted 218 for three from their 20 overs, which was the highest T20 total ever seen at Northampton.
Ben Duckett's sublime 96 from just 54 balls was the highest individual score by any Northants player in a T20 game at the County Ground, surpassing David Willey's 95 in 2015.
Josh Cobb (56) and Alex Wakely (51no) also caught the eye for the Steelbacks as they smashed a huge score.
But Leicestershire refused to give up hope and they somehow got home with two balls to spare thanks to impressive captain Colin Ackermann.
Ackermann, who was dropped on five by Wakely off Nathan Buck, finished unbeaten on 66 from 31 balls, rifling the six that won the match for his team.
Northants were made to pay for a slow over rate, which cost them six runs with Leicestershire needing 26 from 10 balls, and the Steelbacks will now have to recover quickly as they face champions Nottinghamshire at the County Ground on Friday.
The Steelbacks had lost the toss against the Foxes and were put into bat.
And David Ripley's men lost the early wicket of South African star Richard Levi, who was bowled by Pakistan pace man Mohammad Abbas for six.
But Duckett then got into his stride, playing some lovely shots, including a couple of eye-catching reverse sweeps, putting the pressure on the bowlers during the powerplay.
The Steelbacks were to finish the first six overs on 58 for one, with Duckett continually stepping things up as he made his way to 46 from just 21 balls.
Duckett quickly reached his half-century with another four off the bowling of Zak Chappell, with the batsman making it to 50 in just 22 balls.
And Cobb then got in on the act, smashing 22 runs from four Cameron Delport deliveries as the Steelbacks motored past the 100 mark with just nine overs gone.
The 100 partnership came up in just 44 balls, but after passing 50, Cobb went as he holed out off the bowling off Callum Parkinson for 56 to leave the Steelbacks 123 for two.
But it wasn't a breakthrough that put a spanner in the Steelbacks' works as Duckett continued to keep his composure, alongside skipper Wakely.
Northants eventually brought up the 200 with 11 balls to go in what was turning out to be a sensational innings for the home side.
Wakely was striking the ball superbly and he hammered a couple of sixes out of the ground.
Duckett did depart before the end, earning a huge ovation for his 96 from 54 balls.
Wakely reached his half-century, from just 23 balls, from the next delivery and the Steelbacks saw out the innings to conclude on a whopping 218 for nine.
The Foxes got off to a strong start in reply, but they were hit by an untimely dismissal as Neil Dexter was run out by Seekkuge Prasanna for 38.
Delport and Mark Cosgrove continued to find the boundary, but the former departed when he was caught well by Duckett, who raced in off the rope to give Cobb a wicket.
Cosgrove soon followed Delport back to the dressing room as he was run out for 21 and the Foxes now appeared to be drifting towards defeat.
There was a let-off for Ackermann as Wakely dropped a simple catch off Buck at extra cover.
And Ackermann went on to punish Wakely, reaching a fine 50 in style.
But Ben Sanderson finally got a big breakthrough thanks to a wonderful diving catch on the boundary from Graeme White, who ensured Mohammad Nabi would only make 19.
And the same combination got rid of big-hitting Ben Raine for 23 from seven balls.
However, the Steelbacks still had work to do as their slow over rate saw Leicestershire awarded six penalty runs, meaning the Foxes needed 20 from 10 balls to win it.
And the heat was really on for Rory Kleinveldt in the final over as his first delivery was wide and reached the boundary.
He then hit back by bowling Tom Wells, but next man in, Lewis Hill, managed to get off strike with a single and Ackermann smashed the next ball for six to seal a stunning win.
Steelbacks: Levi, Duckett, Cobb, Wakely (c), Crook, Vasconcelos (wk), Prasanna, Kleinveldt, White, Buck, Sanderson.
Leicestershire Foxes: Hill (wk), Ackermann (c), Delport, Cosgrove, Dexter, Nabi, Wells, Raine, Chappell, Parkinson, Abbas.