Ismaila Sarr got new Watford head coach Xisco Munoz off to the perfect start with a winning goal that ended Championship leaders Norwich City's five-game winning run.
Spaniard Munoz, who was appointed last Sunday after Vladimir Ivic's sacking, was in the Hornets dugout for the first time and watched on as his team produced one of their best displays of the season so far.
Sarr scored the game's only goal six minutes before the break, converting Ken Sema's low cross at the far post.
The Canaries, who are still four points clear at the top, had plenty of possession but lacked any real cutting edge - although Teemu Pukki very nearly found himself clean through late on only to be denied by a magnificent last-ditch tackle by Adam Masina.