Harlequins (13) 20
Tries: Dickson, Brown Cons: Clegg 2 Pens: Clegg 2
Newcastle (6) 21
Tries: Gopperth, Tu'ipulotu Cons: Gopperth Pens: Gopperth 3
Tu'ipulotu came on as a replacement seven minutes into the second half
Tane Tu'ipulotu's injury-time try sent Newcastle into the final of the LV= Cup with a 21-20 win at Harlequins.
Quins had the best possible start with a Karl Dickson try in the first minute, before Rory Clegg and Jimmy Gopperth each exchanged two penalties.
Trailing 13-6 at half-time, Gopperth then hauled Newcastle level as he converted his own try, before Mike Brown edged Harlequins ahead again.
But Tu'ipulotu went over with the final play of the game for a famous victory.
Gloucester and Newport Gwent Dragons
contest the second semi-final on Sunday.
Newcastle will then meet the winners at Franklin's Gardens, Northampton on 20 March.
Harlequins coach Conor O'Shea:
"This is incredibly difficult to take. We'll feel very sorry for ourselves for a couple of days and everyone should. It would be the wrong emotion not to.
"We are a good rugby side but we didn't finish our chances. That happens. From time to time you get a match like this. It is great when you win. It is a horrible, horrible thing to take if you lose.
"We will have to stew on this for a couple of weeks because we don't have a game and that will be hard. But you have to lick your wounds, take the pain and move on to your next job. We have five league games to go and that is a way into Europe."
Newcastle coach Alan Tait:
"You can count on one hand as you go through your career the amount of times you win a game like that.
"They had thrown everything at us. I had hardly sat in my seat and they were 7-0 up and Ugo Monye dropped a scoring opportunity as well.
"You ride your luck but I have to praise our lads for the effort they put in. They never say die. They have really battled."
Harlequins: Brown; Williams, Lowe, Turner-Hall, Monye; Clegg, Dickinson; Marler, Cairns, Andress; Kohn, Browne; Fa'asavalu, Skinner, Robshaw.
Replacements: Gray for Cairns (38), Lambert for Andress (67), Vallejos for Fa'asavalu (64), York for Skinner (17).
Not Used: Jones, Moore, Urdapilleta, Camacho.
Newcastle: Tait; Williams, Eves, Fitzpatrick, Bobo; Gopperth, Young; Golding, Thompson, Murray; Hudson, Van der Heijden; Swinson, Wilson, Hogg.
Replacements: Tu'ipulotu for Williams (47), Charlton for Young (57), Shiels for Golding (57), Vickers for Thompson (41), Brookes for Murray (65), M. Wilson for B Wilson (73).
Not Used: Levi, Catterick.
Referee: Nigel Owens (WRFU).