Joshua King's hat-trick helped Watford to a stunning first win under Claudio Ranieri as Everton collapsed in a pulsating Premier League game at Goodison Park.
Tom Davies put the hosts ahead early on but Watford equalised before the break through King from close range.
Former Hornets striker Richarlison put Everton ahead again but Juraj Kucka's header drew Watford level.
King, who didn't score for Everton during his spell there in the first half of 2021, then struck twice inside six minutes before Emmanuel Dennis put the seal on an incredible comeback.
While Watford had never won at Goodison in any competition before this match, Ranieri has happier memories of trips to Everton - this was his fifth consecutive win at the ground.
It is a second successive home defeat for Everton's former Liverpool manager Rafael Benitez and some boos greeted the Spaniard's decision to replace Anthony Gordon with Richarlison in the second half.
He seemed vindicated when the Brazilian struck with a fine diving header just three minutes after coming on, but Benitez then saw his side's lead evaporate in a disastrous final 10 minutes that leaves his side in eighth place.
Watford, having started life under Ranieri with an emphatic defeat against Liverpool, climb the table to 14th - suddenly well clear of the relegation zone.