A George Pisi hat-trick powered Northampton into next week's LV= Cup final at Sandy Park as they defeated Saracens 26-7 at Franklin's Gardens.
Saracens dominated the first half but were left to rue missed chances and a ruled out try, with just a converted score from Ben Ransom to show for their efforts.
Northampton stormed back after the break with Pisi's three tries augmented by five points from Stephen Myler's boot and two penalties by Will Hooley.
The home side started well with James Craig getting over the ball to win the first penalty for 20-year-old fly-half Hooley made it 3-0 after four minutes. Five minutes later Hooley made it 6-0 when a 10 metre lineout drive was brought down by the visitors but for the rest of the half it was all Saracens.
A break from their 22 saw Tim Streather storm up field, but Michael Tagicakibau was caught by Tom Collins eight metres short of the line and his offload fell to the floor.
Two minutes later Saracens' Jack Collins broke on the opposite wing only to be stopped by England full-back Ben Foden, who was making his first start since the end of November after a knee injury.
Young Northampton centre Tom Stephenson limped off early on to be replaced by Myler with Hooley moving into midfield and from then on gaps started to appear in what is the best defence in the competition.
The pressure finally told in the 20th minute as Jackson Wray broke off a five-metre scrum to within a metre of the try-line and Ransom then slipped a tackle to touch down. Scrum-half Ben Spencer converted to put Saracens 7-6 in front.
Northampton's kicking game was not up to scratch whereas Saracens were typically voracious in chasing theirs and it won them a 31st minute penalty, but Spencer missed from 35 metres.
Saracens thought they had a second try when Marcelo Bosch stepped through the Northampton line 10m out but referee Leighton Hodges went to the television match official, who spotted a forward pass from Nils Mordt to Wray in the build-up.
The home side were fortunate and relieved to go into half-time just one point behind. The visitors continued their domination with Streather wrecking a great chance for a second try but Saints won a scrum penalty on their 22 and the momentum changed.
A clever kick from Kahn Fotuali'i brought Northampton two five metre lineout drives that got them onto the Saracens' try-line and at pace Pisi clattered over though it took television replays to confirm it.
Hooley missed the simple conversion to leave the home side 11-7 ahead, though Myler made up for it with a drop-goal on 59 minutes.
Northampton showed Saracens how to take their chances, with Pisi claiming a second try after a terrific Saints kick-chase brought a scrum five metres out and quick ball saw the centre charge over from short range on 65 minutes. Hooley again missed the extras.
Charlie Hodgson came off the bench and intercepted a Samu Manoa pass to almost race home from his 22, but Pisi superbly brought him down five metres short.
With two minutes left on the clock the Samoan completed his man of the match display by blasting through on the cut back to score his third try that Myler converted.