Football's capacity for the dramatic has been remarkably high this season, but the final 90 seconds of this match will surely never be forgotten by fans of both these clubs.
The seven minutes of added-time had almost expired when Leicester's Anthony Knockaert went to ground under the gentlest of contact from Marco Cassetti and referee Michael Oliver pointed to the spot.
Knowing it would put his side into the Championship play-off final, Knockaert himself took it, only to see Manuel Almunia block the shot with his feet. The ball rebounded to Knockaert, but Almunia saved with his hand and the ball was cleared.
Watford's Ikechi Anya picked up possession, found Fernando Forestieri, whose cross was headed across goal by Jonathan Hogg and lashed into the net by Troy Deeney.
Watford boss Gianfranco Zola's faith in Matej Vydra as Deeney's partner up front was vindicated when the Czech scored twice, either side of David Nugent's header for Leicester.
Zola, who after Deeney's winner ran on the pitch and fell over, said: "I felt the ground slip from under me and I've probably pulled something but it's okay. That's the passion, that's why we love this game."