Marnick Vermijl's early goal was enough for Preston to beat 10-man Watford 1-0 at Deepdale and book their spot in the third round of the Capital One Cup.
The game was just eight minutes old when Vermijl reacted quickest to Josh Brownhill's scuffed effort to put the Championship side into an early lead.
And in truth that was never really threatened as Preston more than held their own against Watford who are unbeaten in the Premier League this season and saw debutant Miguel Britos sent off with 15 minutes remaining.
After a scruffy opening Preston took the lead in the eighth minute. The ball fell for Brownhill on the edge of the area who scuffed his shot only for it to fall for Vermijl who slotted the ball home from 12 yards.
Preston skipper Tom Clarke tried an ambitious header from 25 yards after 27 minutes, after latching on to Jordan Pickford's goal kick, but the effort looped into the arms of Giedrius Arlauskis.
Kyel Reid threatened to trouble the Watford defence after 31 minutes, jinking down the left before trying the beat Juan Parades too many times and seeing his cross deflected behind.
Watford crashed out at this stage last season to Doncaster and the closest they came in a disappointing first half was a 45th-minute corner which flashed just above the head of Gabriele Angella before being punched away by Pickford.
Watford almost caught Preston on the counter attack in the 50th minute when Miguel Layun cut in from the left but his low strike from 20 yards was deflected wide by Bailey Wright.
But Preston almost doubled their lead just two minutes later. Alan Browne slipped Brownhill through on goal but he stumbled over the ball before Neil Kilkenny launched a shot from 25 yards which was held low down by Watford debutant Arlauskis.
And Simon Grayson's side pushed forward on the counter attack again after 57 minutes as Reid latched on to a ball over the top, cut inside the defender but fired his left-foot shot wide from the edge of the area.
Preston had an opportunity to make it two in the 71st minute when Ben Watson conceded a free-kick with an inexplicable hand ball. Arlauskis flapped at the delivery but Reid lashed the ball over the bar from the edge of the area.
Watford, who have drawn all three games on their Premier League return, should have equalised a minute later but Fernando Forestieri dragged his shot from eight yards so wide it went for a Preston throw.
And the visitors piled on the pressure, forcing Pickford to acrobatically tip Watson's 74th minute strike from 18 yards over the bar.
Watford debutant Britos was shown a red card in the 76th minute for an off-the-ball collision ahead of a corner, with the suggestion he may have used an elbow, with Watson flicking the corner just wide.