Harlequins booked a place in the Aviva Premiership final after they beat Northampton Saints 25-23 at The Stoop in the first Semi-final.
Harlequins however hit back with a 77th-minute push-over try, awarded to loosehead prop Joe Marler, to snatch victory after trailing 9-12 at the break.
Saints had led 23-15 with ten minutes left to play after a try from scrum-half Lee Dickson, but could not hold out the regular-season winners.
In truth the statistics were heavily in the favour of Quins, who had made a lot less tackles and enjoyed most of the territory throughout.
However, it was Saints holding on to a two-score advantage with three minutes remaining before Marler struck the decisive blow after a strong catch-and-drive.
The opening half was dominated by the place-kicking of the respective fly-halves, but it was the Saints' Ryan Lamb who had the better of the battle, scoring four penalties to Nick Evans' three.
Even though there were more attacking opportunities in the second period, the game continued in the same vein.
Evans kicked three penalties to Lamb's two before Dickson's try seemed to have sealed the win, but another three points from the hosts' pivot reduced the gap and then Marler went over.
Both teams went in with a record of winning four of their past five matches, but it was Quins who scored first through the boot of Evans. His opposite number has a reputation for cracking under the pressure of a big occasion and had a mixed start to this encounter. Although Lamb made several mistakes in the opening ten minutes, he kicked a well-struck penalty to level matters.
Lamb then took Northampton into the lead with a second three-pointer which just snuck over the bar before the hosts' fly-half responded with one of his own almost immediately.
The visiting kicker restored the Saints' advantage, but once again Harlequins hit back with their first real attack of the half.
Maurie Fa'asavalu was particularly prominent making two powerful surges and, under pressure, Roger Wilson cynically slowed the ball down and was subsequently yellow carded. Evans converted the resultant penalty opportunity before a further Lamb three-pointer saw Northampton into a 12-9 lead at the interval.
Despite a disappointing opening period, Quins started the second half in a far more assertive fashion and were rewarded with successive Evans' penalties. But, in a see-saw encounter, the visitors equalised through their fly-half before the same player kicked his sixth from the tee.
Then seemingly came the killer blow. After Saints' winger Vasily Artemyev had made a break, good work from George Pisi and James Downey sent Dickson over.
However, the hosts showed incredible resistance to come back. Evans initially reduced the deficit with a penalty before Marler touched down for a converted score taking Quins to Twickenham.
Final Score Harlequins 25 (9) Northampton 23 (12)
Tries : J. Marler
Penalties: Evans 5
Conversion : Evans
Penalties: Lamb 6
15. Mike Brown , 14. Tom Williams , 13. George Lowe , 12. Jordan Turner-Hall , 11. Sam Smith , 10. Nick Evans , 9. Karl Dickson , 8. Nick Easter , 7. Chris Robshaw (c) , 6. Maurie Fa'asavalu , 5. George Robson, 4. Olly Kohn , 3. James Johnston , 2. Joe Gray , 1. Joe Marler
Replacements : 16. Rob Buchanan , 17. Mark Lambert , 18. Will Collier , 19. Tomas Vallejos , 20. Tom Guest , 21. Richard Bolt , 22. Rory Clegg , 23. Matt Hoppe
15 Ben Foden, 14 Chris Ashton, 13 George Pisi, 12 James Downey, 11 Paul Diggin, 10 Ryan Lamb, 9 Lee Dickson (capt), 8 Roger Wilson, 7 Phil Dowson, 6 James Craig, 5 Christian Day, 4 Mark Sorenson, 3 Brian Mujati, 2 Andy Long, 1 Soane Tonga'uiha.
Replacements: 16 Ross McMillan, 17 Alex Waller, 18 Paul Doran-Jones, 19 Ben Nutley, 20 Teimana Harrison, 21 Martin Roberts, 22 Stephen Myler, 23 Tom May.