Caerphilly booked their place in the Konica Minolta Cup final after pulling off a shock 33-22 win over Aberavon at the Millennium Stadium.
The Castleltown club marched into their first ever Welsh Cup final to follow up their Parker Pen Shield run last year.
"It's absolutely fantastic for the club and the town, I'm so proud of the boys," said coach Roger Bidgood.
Two tries from wing Daran Griffiths looked to have sent Aberavon on their way in the first-half.
But a series of nightmare blunders and gutsy display from the Caerphilly pack sent the Green Army marching on.
Paul Phillips capitalised on a mix-up in the Aberavon defence to keep his side in touch in the first-half.
Richard Lewis and Ricky Price made a has of dealing with Phillips' kick who then pounced to score.
But the damage was done during a blistering six-minute spell soon after the break when Caerphilly scored 17 unanswered points.
Replacement Ian Carter ended a length-of-the field counter attack that started with scrum-half Andrew Pegginton on his own line before lock Mark Workman burst clear from a lineout to go over untouched.
Outside-half Stuart Thomas converted both and added three more penalties in an 18-point haul to clinch their place against Neath in next month's final.
Aberavon: Tries - D Griffiths (2), Daniel Hawkins. Cons - J Davies (2). Pen - J Davies.
Caerphilly: Tries - P Phillips, I Carter, M Workman. Cons - S Thomas (3). Pens - S Thomas (4).