Joshua King's hat-trick fired Watford to a spectacular first win under Claudio Ranieri as Everton subsided late on at Goodison Park.
Tom Davies gave the hosts the lead within the first five minutes after some impressive wing play from Demarai Gray.
But ex-Toffee King drew the sides level less than 10 minutes later from close range.
Richarlison's diving header gave the Merseysiders the advantage again midway through the second half but Juraj Kucka bulleted home a corner to restore parity.
And it was King, who failed to score a single goal during his half-season stint at Everton, that turned the game on its head with two further strikes in six minutes, leaving the home side and their supporters stunned.
Emmanuel Dennis added insult to injury in stoppage time when calmly slotting home Watford's fifth of the contest to round off a remarkable afternoon for the Hornets.
The sensational scoreline sees Ranieri's side move into 14th place, while Everton drop out of the European spots.
And the newly-appointed Watford boss was thrilled with the result.
Ranieri said: "It was important to show the character, the strength and to react to the result.
"We were playing well but suddenly we score in nine minutes three times — it was amazing.
"I'm very happy because last week I didn't have King and today I had King.
"King made a fantastic match. Not only because he scored a hat-trick but because he held the ball up, he won all the duels — he was fantastic in every action."
Did you know?
Everton suffered their first ever home defeat against Watford in any competition today.
Watford scored five goals in an away Premier League game for just a second time.
Watford came from behind to win an away Premier League game for the first time since January 2019.
Watford’s King became just the third player to score a Premier League hat-trick against a side he's previously played for in the competition.
Tom Davies' opener for Everton was the fastest Premier League goal the Toffees have scored at Goodison Park since April 2017.
There were just 172 seconds between Richarlison entering the field of play as a substitute and putting Everton 2-1 ahead.
There were just 88 seconds between Juraj Kucka’s equaliser for Watford and King scoring the Hornets’ third goal.
There have been four yellow cards brandished in the Premier League this season for diving, two of which have gone to Everton players.