Northampton stunned Saracens 27-13 at Allianz Park to reach their first Aviva Premiership final - and set-up a title showdown with east Midlands rivals Leicester.
The Saints had lost all five of their previous Premiership semi-finals but they roared into a 17-0 half-time lead with quick-fire tries from Brian Mujati and Jamie Elliot.
Saracens did engineer a try from Duncan Taylor when Elliot was in the sin-bin but Northampton's grip on the game was vice-like and Stephen Myler's second penalty, on top of his three conversions, wrapped up a famous victory.
Northampton's power engineered the breakthrough for the opening try, with a lineout drive try for Mujati, and the Saints raced into a 17-0 lead by the interval. Calum Clarke claimed the lineout and Northampton drove for the line, with Mujati stretching out an arm to touch down for a try that was awarded by television match official Graham Hughes.
Myler converted from wide on the right and, just two minutes later, he was successful from the left wing after Northampton scored a scintillating second try through Elliot.
Samu Manoa laid the foundation with a crunching tackle on Chris Ashton and Northampton launched a counter-attack from the turnover, with Dylan Hartley and Christian Day combining to release Elliot with the scoring pass.
And with an hour gone, Northampton again went over through Van Velze, who galloped over for Northampton's third try, having come on as a replacement for Courtney Lawes.
Northampton secured lineout ball on the Saracens 22, Hartley carried the ball forward and then recycled quickly for Lee Dickson, who spotted an opening down the short side and released Van Velze.
Northampton were reduced to 14 men when Elliot was sin-binned for taking Farrell out in the air and Saracens took advantage, eventually working space for Taylor to score the try.