Graham Alexander's penalty earned Preston a win against a Watford side who have now gone five league games without a victory.
Watford had defender Sean Dyche sent-off for two bookable offences and he was the guilty party in conceding the spot-kick.
He brought down Chris Lucketti for the penalty and was yellow carded for his protestations before being dismissed for a hefty challenge on Preston keeper Jonathan Gould which led to a 16-man brawl.
The match had been evenly poised up until the penalty which Alexander emphatically powered home.
Hornets striker Jason Norville had a great chance to level after 27 minutes but he failed to make a proper connection from six yards out.
Dyche's rush of blood to the head then left the hosts with a mountain to climb.
Preston used their extra man to assume the lion's share of possession and Watford were forced into playing on the break.
But the visitors still struggled to create clearcut chances and were almost caught out when Norville volleyed over when well placed moments before the break.
A superb tip over the bar from Watford keeper Richard Lee prevented Marlon Broomes doubling Preston's lead with a 25-yard drive straight after the restart.
The home side maintained a threat of their own and almost levelled in the 55th minute when Gould saved a Gifton Noel-Williams header right beneath his crossbar.
Gould's opposite number Lee was able to keep Watford in the hunt when Eddie Lewis put his close-range effort too close to the keeper.
But Preston had already done enough to secure their second 1-0 away win in four days.
Watford: Lee, Ardley, Dyche, Brown, Robinson, Hand, Fisken, Mahon, Glass, Noel-Williams, Norville.
Vernazza, McNamee, Doyley, Godfrey.
Preston: Gould, Alexander, Broomes, Lucketti, Jackson, Cartwright, McKenna, O'Neil, Lewis, Koumantarakis, Cresswell.
Subs: Lucas, Rankine, Mears, Skora, Lynch.
Referee: J Ross (Essex).