Longford 1-12 Dublin 0-09
Hosts Longford finished with a flourish to seal a surprise Bord na Mona O'Byrne Cup final place at holders and All-Ireland champions Dublin's expense.
The massive underdogs never trailed at Pearse Park and outscored Dublin by 1-03 to 0-01 in the decisive closing 10 minutes to seal a final clash with Meath next weekend.
Jim Gavin's Dubs had fought back from a three-point half-time deficit to trail by just one with 20 minutes to go and that margin remained after an hour.
But, instead of closing out the game as anticipated, the Dubs wilted as Longford finished with real gusto and, crucially, found the net through Liam Connerton six minutes from time.
Robbie Smyth and Francis McGee also weighed in with three points apiece on a memorable day for the midlanders.
Dublin, who began last year's all conquering season with a first O'Byrne Cup success since 2008, were installed on low odds to return to the final.
But from the opening minutes it was clear that Longford meant business as they attacked with purpose and power, taking an early two-point advantage.
Dublin did get back to level terms at 0-02 apiece after 11 minutes following points from Darren Daly and Paddy Andrews but the respite was brief.
McGee kicked his second of the afternoon before a Brian Farrell score sent Longford two clear again.
The margin was out to three points after 23 minutes following Longford points from James McGivney and Smyth, 0-06 to 0-03.
New manager Denis Connerton, in his second spell as manager, was without Dublin-based attacker Brian Kavanagh but Longford's forward unit still looked sharp and all three members of the full-forward line made the score sheet by half-time when they led 0-07 to 0-04.
Smyth closed the scoring for the half with a thunderous free from 55 metres out, leaving the large Dublin support in the 2,000 strong crowd silent.
Goalkeeper Stephen Cluxton made his first appearance of the season for Dublin while All-Ireland finalists Jonny Cooper, Philly McMahon, James McCarthy, Denis Bastick, Dean Rock and Paddy Andrews also started.
It was Dublin's strongest team of the season so far having drawn with Wexford in Group A before overcoming IT Carlow and DCU.
Among the fringe talent attempting to impress ahead of the Allianz league which begins on Saturday week, at home to Kerry, were full-back Jarlath Curley, Ciaran Reddin, Shane Carthy (St Vincent's) and Con O'Callaghan.
Both sides continued to battle courageously in heavy conditions with rain falling throughout. Philly Ryan became the 31st Dublin player to feature in their four-game campaign when he was introduced with 20 minutes remaining.
The visitors reduced the margin to a single point at that stage, thanks in part to three points from Rock, two from frees, which made it 0-09 to 0-08.
That was as good as it got for Dublin though as Longford finished impressively, holding the Leinster champions to just one more point for the remainder of the contest, from Reddin.
Longford led by three when Connerton essentially sealed victory, doubling their advantage with his 64th-minute goal.
McGee's ball in from the right wing was well held and then laid off by Michael Brady for Connerton to poke in from close range with a soccer-style finish.
Longford: P Collum; F Battrim, B Gilleran, CP Smyth; A Dalton, D Brady, D Masterson; M Brady, B Farrell (0-01); B McKeon (0-01f), J McGivney (0-01), D Gallagher (0-01); R Smyth (0-03f), F McGee (0-03, 1f), M Hughes (0-01).
Subs: P McKeon (0-01) for Hughes, D McElligot for Farrell, L Connerton (1-00) for Smyth, C Shields for P McKeon (BC), P McGee for F McGee.
Dublin: S Cluxton; J Cooper, J Curley, P McMahon; J McCarthy, J Small, D Daly (0-01); C Reddin (0-02), D Bastick; T Brady (0-01), D Rock (0-03, 0-02f), S Carthy; C Costello (0-01), P Andrews (0-01), C O'Callaghan.
Subs: N Scully for Bastick, P Ryan for O'Callaghan, C Mullally for Carthy, M Deegan for Andrews.
Referee: J Hickey (Carlow).