Harry Smith ensured that whilst Braintree Town's history-making run in the Emirates FA Cup came to an end, his own fairytale introduction to professional football continues with a first ever senior hat-trick to book The Lions place in Monday's Third Round draw.
Smith, who was playing non-League football himself for Folkestone Invicta last season, produced clinical finishes on 17, 20 and 47 minutes to ensure that there was no giant-killing act by National League outfit Braintree as he took his goals tally to six in six starts for The Lions.
In a ding-dong Den tussle high on energy and enthusiasm and at times played at a frenetic pace, the visitors gave their all in front of a 3,345 crowd, of which 315 had made the journey from Essex.
Those away supporters were in dreamland when Braintree, playing in fluorescent orange, had their hosts blushing red as they took a 16th-minute lead. A slip by Sid Nelson was taken full advantage of by Michael Cheek, who powered through in a central position and thumped the ball beyond the reach of Jordan Archer.
The travelling fans were still celebrating when Millwall hauled themselves level within 60 seconds of going behind. Lee Gregory did well to hold off his marker, spun on the edge of the penalty area and threaded the ball to Smith, who made no mistake from close range.
The Lions got their noses in front three minutes later with Smith's second goal of the game, this time a crisp half-volley after Gregory chalked up assist No.2 by steering Shane Ferguson's corner into his path at the back post.
Harris' men looked to be in control when Ferguson drilled home a third beyond the reach of keeper Sam Beasant after Gregory's sublime pass had picked him out on the left. Ferguson shimmied his way past a couple of defenders and made no mistake with a crisp finish into the far corner with 23 minutes on the clock.
Braintree, at this stage, were looking vulnerable every time Millwall progressed beyond the halfway line and decided that attack was going to be their best form of defence as they began to ask questions of their own.
A neat passage of play between Sean Clohessy and Simeon Akinola offered further encouragement for the visitors, and they were right back in the mix 11 minutes before the break when Jack Midson poked the ball home from a central position after a swift break down the right.
Smith and David Worrall both went close as The Lions looked to restore their two-goal cushion, but it was down to the athleticism of Archer that Braintree didn't go in level, as the keeper somehow kept out Monty Patterson's goal-bound effort.
Millwall didn't have long to wait - two second-half minutes to be precise - before adding a fourth goal. A corner from Worrall caused confusion in the penalty area and when the ball eventually broke to Smith, he made no mistake with a sweeping finish to complete his treble.
Akinola sent a sweetly struck half-volley over the bar as Braintree tried to conjure a quick response, and it took a super save from Archer to prevent a third goal for the visitors, spreading himself low to push away Cheek's effort at point-blank range.
Smith was a whisker away from adding his fourth - and Millwall's fifth- with a downward header from Ferguson's hanging cross, but the ball dropped inches wide of the upright.
But as the game began to open up, back came Braintree and Akinola hit a thunderous 18-yarder just over as the non-League side refused to give up on their dream of producing an upset.
Ian Gayle became the first player to be treated for cramp with 21 minutes remaining and he was ultimately replaced as the game entered the final 10 minutes after requiring treatment again.
Lions sub Aiden O'Brien, a 78th-minute replacement for hat-trick hero Smith, hit a low shot that was blocked and then fellow sub Fred Onyedinma nearly made an instant impact with a fierce drive that was diverted by the boot of a Braintree defender as The Lions searched for a fifth goal to really put the result beyond reach.
At the other end former Southend United striker Lee Barnard tried his luck from distance, as did goal-scorer Cheek, as the visitors looked to set up a grandstand finish.
But it was Millwall who had the final with the last kick of the game - a deft flicked finish by O'Brien from barely four yards after fellow sub Onyedinma and Worrall had combined to set him up with a gilt-edged opportunity.
Archer, Martin, Williams, Worrall, Gregory (Onyedinma, 86'), Romeo, Nelson (Craig, 64'), Butcher, Webster, Ferguson, Smith (O'Brien, 78'). Subs not used: King, Cummings, Thompson, Abdou.
Smith (17, 20, 47'), Ferguson (24'), O'Brien (90+4')
Beasant, Okimo, Lee, Gayle (Barnard, 84'), Midson, Isaac (Maybanks, 88'), Akinola, Jules, Cheek, Patterson (Matthews, 62'), Clohessy. Subs not used: Strizovic, Williams, Hall-Johnson, Corne.
Isaac (58'), Gayle (60')
Cheek (16'), Midson (34')
3,345 (away 315)