Watford were down and out when Gerard Deulofeu entered the fray in the 66th minute with a point to prove after losing his place in the Hornets' starting line-up.
The Spaniard, who rose through the ranks at Barcelona, could have moaned and groaned about being dropped but he instead made the difference at Wembley.
Here, Sportsmail looks at how the game changed in Watford's favour following Javi Gracia's substitution with 24 minutes of normal time remaining.
66th minute - Gerard Deulofeu replaces Will Hughes
Gracia feels the need to change his attack as Troy Deeney and Andre Gray appear to be slightly ostracised in Watford's attack. Will Hughes and Roberto Pereyra are failing to link the midfield with the forward line so Gracia turns to Deulofeu. The former Everton man must feel hard done by that he's not starting but he knows he can make a difference with his side 2-0 down with 24 minutes remaining.
77th minute - Hornets finally find their wings
Watford had no width before the introduction of Deulofeu as Hughes, Pereyra and Abdoulaye Doucoure — who were playing in a diamond with Etienne Capoue at the base — all like to play inside rather than on the wings. Deulofeu made an instant impact by staying wide to cause Wolves problems.
79th minute - Deulofeu pulls one back with stunning strike
Deulofeu scores a goal which his former Barcelona team-mate Lionel Messi would be proud of. The Spaniard manages to lob the ball past John Ruddy and into the net with very little backlift. He makes Watford fans and his colleagues believe again when it looked like they were going to fall at the final hurdle before a potential FA Cup final with Manchester City.
88th minute - Jose Holebas squanders chance
Sometimes chances fall to the wrong player and that's the case in the 88th minute. Holebas isn't exactly the most prolific star on Watford's books with just six goals in 103 appearances for the Hornets and he fails to show composure in front of goal. It looks like Watford's hopes have gone...
92nd minute - Watford are given lifeline
Leander Dendoncker gives Watford a last-minute lifeline when his side are just moments away from setting up a date with Manchester City. The Belgian is extremely clumsy as he takes out Deeney. Michael Oliver is left with no choice but to point to the spot. Deeney is given a few minutes to regain his composure as Oliver checks the decision with VAR.
94th minute - Troy Deeney converts penalty
Gracia said he was sure Deeney would slot home his penalty and the Watford captain did not let him down. The 30-year-old hit it straight down the middle with force – leaving John Ruddy clutching at thin air as he dived to his right-hand side.
102nd minute - Wolves make attacking change
Nuno Espirito Santo throws on Adama Traore in hope that the pacy winger can run at Watford's tiring defence. The attacking change means there's more space in the wide areas high up the pitch as Traore's struggles with defensive responsibility. You can see Nuno's thinking but will it cost him?
104th minute - Deulofeu puts Watford in front
That's exactly what Watford were missing before Deulofeu came off the bench. Gray plays a one-two with Deulofeu. The Spaniard then runs with the ball on the right wing and slots the ball home to put his side in the lead for the first time at Wembley. Now it's Watford's turn to try to hold onto a lead.
112th minute - Deulofeu forced off with injury
The substitute has to be subbed but Watford fans won't care as he's done his job. The Hornets have eight minutes left to run down the clock after their star man is forced off with an injury. The Spaniard shares an embrace with his manager.
122nd minute - Full-time whistle goes
Watford set up an FA Cup final date with Manchester City by completing a shock turnaround. Hornets fans, players and staff are all smiles as they celebrate at Wembley. Now attentions turn to facing Pep Guardiola's men...