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.