An early goal from Marnick Vermijl proved enough for Preston North End to claim the scalp of Premier League new boys Watford and book their place in the third round of the League Cup.
Having lost once so far in 2015-16, Championship side Preston produced a confident and accomplished display against Quique Sanchez Flores' men, who are still searching for a first win of the season.
To compound a miserable night for Watford, Miguel Britos was sent off late on and Flores' team must now try and regroup ahead of a daunting visit to league leaders Manchester City on Saturday.
Goalkeeper Giedrius Arlauskis, one of three Watford debutants along with Nathan Ake and Miguel Britos, was forced to pick the ball out of his net after just eight minutes as North End made a blistering start in front of a sparse crowd at Deepdale.
Josh Brownhill's initial shot was blocked, but the ball fell kindly for Vermijl and the on-loan Sheffield Wednesday defender calmly scored from close range.
Ake, meanwhile, was involved in the next incident of note when former Watford man Joe Garner went to ground under his challenge in the box, but referee Stephen Martin waved away the home side's penalty appeals.
Having taken their time to recover from that early setback, Watford steadily improved as the half wore on and a stretching Matej Vydra volleyed over just short of the half-hour mark.
Fernando Forestieri was similarly off target for the visitors with a 35th-minute free-kick and Miguel Layun also failed to keep his well-struck, long-range effort down in what was the final action of the first half.
Layun was again involved as Watford began the second half brightly, seeing his low shot deflected wide following the kind of swift counter-attack that proved so effective for the men in yellow during last season's Championship promotion campaign.
But Preston's response was immediate, with a last-ditch tackle from Juan Carlos Paredes required to foil Brownhill as he burst into the box and Garner then drawing a decent save from Arlauskis.
Former West Ham winger Kyel Reid was next to go close for the home side, beating both Ake and Paredes before firing narrowly wide after 55 minutes.
Flores duly threw bodies forward in a desperate search for an equaliser, but other than a half-chance missed by Forestieri, Watford rarely threatened.
And once Britos received a straight red for elbowing Bailey Wright, Preston were able to comfortably see out the final moments and seal an impressive win.
