Cyle Larin proved why Southampton boss Tonda Eckert rates him as the best striker in the Championship as his double in a 5-1 victory shattered Millwall's perfect start to the season.
Mark Sykes had given the Lions an early lead, but Larin's brace along with Finn Azaz's tap-in, Leo Scienza's penalty and Tom Fellows' first Saints goal blew the visitors away.
The Canadian has scored four goals in the first three league matches of the season to take his Saints tally to 12 goals in 19 Championship appearances.
Southampton have now wiped out their four-point deduction for 'Spygate', while Millwall were beaten for the first time this season.
Alex Neil's side came into the match top of the Championship, and for the opening 15 minutes showed just why they had back-to-back wins to start the campaign.
Tairyk Arconte forced a save out of Daniel Peretz, before Josh Coburn headed wide.
The one-way traffic was rewarded when Sykes picked up on a half-clearance on the edge of the box before driving home, via a deflection. It was the Irishman's first goal for the club since arriving on a free transfer from Bristol City this summer.
But from there, the hosts dominated in every facet of the pitch - starting with Ryan Manning crashing a short corner against a post.
The leveller came in the 25th minute from Larin. Manning's deep free-kick was powerfully met by the Canadian, with the frontman continuing his 100 per cent scoring record in the league this season.
The hosts completed the turnaround 13 minutes later when Scienza skipped past Jake Cooper, darted towards the byline, clipped the ball over goalkeeper Max Crocombe and gave Azaz an unmissable tap-in.
Despite the pulsating resurgence from Saints, Millwall should have been level at the break, as Kyrell Lisbie fired against a post when offered up an open goal by Arconte on the counter.
Returning hero James Ward-Prowse's introduction for the start of the second half further buoyed the home fans, and they were sent into raptures when Azaz was tripped in the box by Cooper, with Scienza dispatching the penalty with aplomb.
The icing was put on the cake with the best goal of the lot in the 62nd minute. Larin lifted the ball over Cooper's head before volleying into the bottom corner from the edge of the box.
He should have sealed a hat-trick but Crocombe - playing in goal instead of the injured Lukas Jensen - stayed big one-on-one.
Southampton did get their fifth when Fellows celebrated a year since arriving at St Mary's by firing past Crocombe, as the home side eased to a 12th home match unbeaten in the league.
Southampton's Tonda Eckert:
"It doesn't hurt to have some points on the board. We are chasing. We knew we needed to chase this season and we have shown we are capable of doing that.
"That second finish [from Larin] is just class. There are some things you do as a striker that you do when you are full of confidence, and you don't do when you don't have it.
"He is able and willing to take responsibility for the team. There is so much he does off the pitch that is not seen publicly that makes him so good to have in the building.
"There was a chance to score the hat-trick, but he didn't take it…so I took him off!"
Millwall's Alex Neil:
"I thought it was a strange game in that we were very good for 25 minutes. We took the lead and should have scored another while we were on top.
"The goals were so poor from our point of view. We were lacking that resilience and grit that we always have in the last 20 minutes of the game.
"I am bitterly disappointed at how the game ended up."
