Watford 3-1 Leicester City (3-2 on agg.)
TROY DEENEY`s 98th minute winner will take Watford to Wembley for a shot at the Premier League, just seconds after Anthony Knockaert had missed an injury time penalty for the Foxes.
Matej Vydra put Watford ahead with a stunning left-footed volley after 15 minutes before David Nugent nodded Leicester level on the day from a corner just four minutes later.
In the 65th minute Vydra scored his 22nd of the campaign, tucking under Schmeichel from inside the penalty area to make it two-all on aggregate.
Deep in injury time, Knockaert took a fortuitous tumble inside the penalty area and stand-in referee Michael Oliver awarded the spot kick. Knockaert missed and Watford broke to secure the win in the most dramatic fashion possible.
Gianfranco Zola made three changes to his starting XI, bringing Deeney, Cristian Battocchio and Matthew Briggs in for Alex Geijo, Jonathan Hogg and Daniel Pudil.
Foxes` boss Nigel Pearson stuck with the same side victorious in the first-leg, meaning Jeffrey Schlupp continued at left-back in place of Paul Konchesky.
The Foxes shot out of the blocks and won a string of early corners with Ritchie de Laet coming the closest with a near-post effort blocked by Deeney and Joel Ekstrand.
Watford eventually managed to escape their own penalty area after six minutes, Deeney picking out Vydra who slotted a low effort narrowly wide of Kasper Schmeichel`s right-hand post from the edge of the penalty area.
Schmeichel was called upon once more a few minutes later when Lloyd Doyley hoisted a pass onto the chest of Deeney who powered past Michael Keane before unleashing a powerful low drive which Schmeichel palmed away.
In the 15th minute Vydra pulled Watford level on aggregate with his first league goal in 15 outings, coolly volleying Marco Cassetti`s lofted pass beyond Schmeichel with a first-time left-footed volley from an acute angle.
Parity was short-lived as Leicester`s goal hero from the first leg, Nugent, nodded the Foxes back in front just four minutes later as he got above Lloyd Doyley to steer home from a corner.
Zola`s side were continuing to make chances with Deeney testing Schmeichel after half an hour with a firm 20-yard effort before sending a bicycle-kick comfortably wide from an Ikechi Anya cross five minutes later.
Half-time: 1-1 (1-2 on agg.)
Six minutes after the re-start Watford nearly levelled on aggregate when Battocchio showed some excellent footwork to manoeuvre enough space between two Leicester defenders to fire a low shot narrowly wide with the aid of a deflection.
On top, Watford should have pulled level once more just before the hour when Briggs danced past Ritchie de Laet and sent a low cross in for Almen Abdi who controlled before seeing his shot deflect off of Keane for a corner.
From the resulting corner Cassetti turned an effort towards goal from the near-post; the Hornets appealing for handball which looked harsh at close-range.
On 65 minutes Zola introduced Fernando Forestieri in place of Lloyd Doyley and within 60 seconds the extra impetus had paid off as Vydra drew Watford level once more, controlling Deeney`s pass and rolling under Schmeichel.
Battocchio was next to try his luck with 18 minutes to play, collecting Forestieri`s pass from the left & firing a swerving low effort into the grasp of a grateful Schmeichel.
Leicester refused to lie down and die however and City substitute Harry Kane came close to putting his side back in front when he drilled a low shot into at Manuel Almunia at the near-post.
With just 13 minutes left on the clock, Deeney momentarily looked to have put Watford in front for the first time when he raced onto Vydra`s through-ball, tucking under Schmeichel. The goal was ruled out for offside however.
Deep into the additional four minutes Leicester were awarded a penalty when Knockaert took a tumble in the penalty area after minimal Cassetti contact.
The Frenchman stepped up to take the penalty but was denied by Almunia before the Spaniard sprung up to block once more.
Watford shot up the other end and from Jonathan Hogg`s header back across the face of goal, Deeney arrived to bundle past Schmeichel to send Watford to Wembley in barely believable circumstances.
Full-time: 3-1 (3-2 on agg.)
Watford: Almunia (c); Doyley (Forestieri), Ekstrand, Cassetti; Anya, Abdi, Chalobah, Battocchio (Hogg), Briggs; Deeney & Vydra.
Unused Subs: Eustace, Hall, Pudil, Geijo & Bond (GK).
Leicester City: Schmeichel; De Laet, Morgan (c), Keane, Schlupp; Knockaert, James, King, Dyer (Drinkwater); Wood (Kane)& Nugent.
Unused Subs: Konchesky, Marshall, Vardy, Moore & Logan (GK).
Attendance: 16 142
Referee: Mr M Oliver