Vakoun Bayo scored four second-half goals in an astonishing individual display as Watford netted five times after the break to crush Sheffield Wednesday and end a run of five consecutive away league defeats.
The Hornets had not won away from home in the Championship since their opening-day success against Millwall and were second best before the break - and fortunate to be going in level at 1-1 despite taking the lead through a neat close-range finish by Ryan Porteous.
Michael Smith’s clever volley deservedly levelled for the Owls, but Tom Cleverley’s side destroyed their hosts in the second half.
Penalties by Tom Ince and Bayo, in the space of six minutes, put Watford in control.
Bayo then headed home home a fourth away goal before Pol Valentin offered the faintest hope of an unlikely comeback with a far-post volley that got the score back to 4-2 with eight minutes remaining.
An exquisite chip completed Bayo’s treble and he took his season tally from one in 11 league games to five in 12 to seal an incredible victory and take the Hornets up to fifth in the Championship table.
There was no sign of the extraordinary second half to come after a first period where Wednesday had the better of things and restricted their opponents to a solitary attempt on target.
The Owls, chasing a first win at Hillsborough in five games, were the stronger starters and D'Shon Bernard twice missed decent headed chances.
The defender nodded an effort wide from Marvin Johnson’s cross and, soon after, he was also off target after a smart run helped him create space from a Shea Charles free-kick.
Watford led against the run of play, however, Porteous turning the ball home at the near post after some fine wing play by Kwadwo Baah on the left.
Smith’s cleverly controlled volley levelled the scores, but Ince’s confident penalty - after Bernard handled the ball while attempting a diving header - sent James Beadle the wrong way to restore Watford’s lead seven minutes after the interval.
The Vakoun Bayo show
But Ivory Coast international Bayo then took centre stage, scoring four goals in a remarkable 30-minute spell.
58 mins - Bayo doubles his season tally. The attacker earns a spot-kick when he is felled by Dominic Iorfa and scores his first goal with a coolly-taken penalty that he blasts down the middle.
67 mins - Two in nine minutes. Bayo heads in a simple second at the far post from an inviting right-wing cross by Giorgi Chakvetadze.
84 mins - The pick of the bunch. Bayo displays all his quality as he races onto a pass on the right by Festy Ebosele and scores with the most divine of dinked finishes over keeper Daniel Bachmann.
88 mins - A simple conclusion. The fourth and final goal arrives with a far-post shot that finds the roof of the net following a fine left-wing centre by Yasser Larouci.
'I hope he kicks on from this'
Sheffield Wednesday boss Danny Rohl told BBC Radio Sheffield: "It's a big disappointment. After the first half, I never thought we would lose this game. We controlled the first 45 minutes in all phases and came back fantastically after we conceded the first goal.
"But it’s not about 45 minutes it’s about 90 minutes. It was horrible to watch the five goals we conceded in the second half. That is not the way I want to see my team.
"It’s hard to see two faces during the game. One was how we want to play with power and energy and pressing moments where we carried the ball and in the second half...I cannot accept the second half.
"We didn't work and didn't cover after the two penalties. And then we conceded two goals at the second post - I cannot accept this today."
Watford boss Tom Cleverley told BBC Three Counties Radio: “We showed real personality to complete in the first half through a not-so-good performance. And then we showed our class and took control of the game with the ball in the second half.
“We were organised without the ball and showed good structure. And in possession we were absolutely fantastic. We were confident and connected. Everyone wanted the ball and we showed real class in the final third.
“I hope Bayo kicks on from this now because he does an amazing job for the team.
“The players took ownership of Bayo taking penalty. I want leaders on the pitch. It was their decision and look how well it turned out. He took confidence from that and went on to score three more.
“The fans didn’t stop singing for the whole game. I was happy the five goals went in at their end in the second half.
“The away performances have not been as shocking as it has looked on paper; we have just had three really poor performances. But we need to back this performance up."