Troy Deeney's 96th-minute goal fired Watford into the npower Championship play-off final in the most dramatic fashion at Vicarage Road, winning 3-1 on the day and 3-2 on aggregate.
The Hornets' promotion hopes looked over when, 60 seconds earlier, Leicester were awarded a penalty.
But Manuel Almunia pulled off a stunning double save to keep out Anthony Knockaert's spot-kick, and also the rebound.
And as Watford broke up the other end the ball fell to Deeney in the penalty area to crash into an empty net and spark wild scenes of celebration.
Hornets boss Gianfranco Zola was sent tumbling to the turf as jubilant players, staff and fans charged on to the pitch.
Watford trailed 1-0 from the first leg and twice levelled the tie on aggregate through Matej Vydra, either side of David Nugent's header.
And after Deeney's amazing winner Zola's side will now face Brighton or Crystal Palace for a place in the Barclays Premier League on Monday 27 May.
Vydra had not scored in his last 12 games but ended his drought in sensational style in the 15th minute.
Marco Cassetti chipped the ball over the top and Vydra, on the run, let it drop over his shoulder before crashing a superb angled volley across Kasper Schmeichel and into the net.
That levelled the tie on aggregate but Leicester got their noses back in front just four minutes later, Nugent out-jumping Lloyd Doyley at the far post to nod in Matty James' corner.
The Foxes could have had another moments later but Knockaert skied his effort.
Watford piled forward after the interval with Cristian Battocchio tip-toeing through a couple of challenges on the edge of the box before shooting just wide.
Then Vydra's lay-off gave Almen Abdi an opportunity 15 yards out but his drive also flew the wrong side of the post.
The goal they desperately needed came in the 65th minute when Vydra played a neat one-two with Deeney just inside the area and sidefooted his second past Schmeichel.
Deeney had the ball in the net with 12 minutes remaining after running on to Vydra's through-ball but an offside flag curtailed his celebrations.
But with extra time looming Cassetti nudged Knockaert to the ground in the area and referee Michael Oliver pointed to the spot.
Knockaert was already celebrating in front of Leicester's fans but that proved premature when Almunia saved his penalty with his legs and got up in time to parry the rebound.
And as if that was not dramatic enough, Watford charged back upfield and moments later Jonathan Hogg's header back across goal was touched on by Vydra and Deeney gleefully slammed in the winner.