Joshua King's hat-trick helped Watford to a stunning first win under Claudio Ranieri as Everton collapsed 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 then struck twice inside six minutes before Emmanuel Dennis put the seal on an incredible comeback.
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