Shaun Hutchinson's 84th-minute header sealed Millwall's place in the Sky Bet League One Play-Offs on Sunday afternoon following a thrilling 4-3 victory at The Memorial Stadium against Bristol Rovers.
Lee Gregory handed The Lions the lead as early as the 6th minute, with Tony Craig doubling the advantage on 26 minutes from close range.
Ex-Lion Jermaine Easter reduced the arrears just seven minutes later, but Gregory netted once more to restore the two-goal advantage just two minutes later.
Chris Lines and Billy Bodin got The Gas back into and levelled the game respectively, but Hutchinson rose to nod home six minutes from time to end a ding-dong in the West Country.
Neil Harris made five changes, some enforced, from the side that were defeated 3-0 by Oxford United at The Den last Saturday. Shaun Cummings, Hutchinson, David Worrall, Shane Ferguson and Jimmy Abdou came into the starting line-up at the expense of Mahlon Romeo, Jake Cooper, Jed Wallace, Aiden O'Brien and Ben Thompson.
The Lions made the early chances, but the ball would not drop in front of goal for the strike partnership of Gregory and Steve Morison to get a clean shot away. Ferguson mustered Millwall's first attempt of the afternoon, but Joe Lumley palmed the Northern Irishman's effort away.
The visitors piled on the pressure and got their just rewards in the 5th minute, however, as Gregory turned home from close range from a Shaun Williams corner-kick.
Bristol Rovers' first chance of the afternoon came through Ellis Harrison, but Hutchinson was on hand to make a heroic block as the striker shaped to shoot from inside the penalty-area.
On 21 minutes, Gregory's lay off found Worrall on the right-hand side. The winger's delicious cross found the onrushing Abdou, but the midfielder could only head over the crossbar.
Five minutes later, The Lions doubled their lead through the unlikely source of captain Craig. Another corner-kick was swung in and after Morison flicked on, the left-back was perfectly placed to poke home from a couple of yards out.
On 33 minutes, The Gas found a way back into the game. Harrison's shot deflected off Hutchinson and onto the post, with the ball falling kindly for ex-Lion Easter, who powered home to halve the deficit.
However, just two minutes later, in an incredible encounter, Millwall regained their two-goal deficit through Gregory. The goal-scorer controlled a long ball forward well and sent Morison down the right-hand channel. The striker's inch-perfect cross found Gregory, who guided the ball past Lumley to make the score 3-1.
The goals kept flowing as on 42 minutes, Lines brought The Gas back to within one with a sublime strike from the edge of the penalty area. Williams and Morison had Millwall's final efforts of a breathless first-half, but their shots from 25-yards out sailed over the crossbar and were saved respectively.
Just one minute into the second-half, Rovers looked as though they would level the scoring - however, an outstanding clearance from Craig kept The Gas at bay.
Bristol Rovers, with the swirling West Country wind behind them, enjoyed large spells of possession as the second-half ticked towards half-way, but The Lions' defence stood firm.
Harris introduced Fred Onyedinma and O'Brien into the play as Millwall looked for differing options in attack to hit Rovers on the break, but it was to be the home side that would level the scoring through Bodin's free-kick on 75 minutes.
Cummings' driven effort had Lumley worried on 79 minutes, but the goalkeeper dealt with the right-back's shot at the second attempt after an initial palm.
Six minutes from time, The Lions nudged themselves in front once more, though, through Hutchinson's bullet header. Ferguson's sumptuous cross found the defender around 12-yards out, with the ex-Fulham man crashing the ball into the net.
After nearly six minutes of stoppage-time, referee Darren Handley blew time on a contest that had swayed back and forth since the very beginning.
The Lions will now take on Scunthorpe United over two legs in the Play-Off Semi-Finals, with the first-leg taking place at The Den on Thursday night.
Bristol Rovers: Lumley, Leadbitter, Brown, Lockyer, Harrison (Broom, 85'), Lines, Easter (James, 80'), Moore, Bodin, Sinclair, Clarke-Salter. Subs not used: Mildenhall, Mansell, Clarke, Partington, Burn.
Scorers: Easter (33'), Lines (42'), Bodin (75')
Millwall: Archer, Cummings, Hutchinson, Craig, Williams, Worrall (Onyedinma, 63'), Gregory (O'Brien, 68'), Webster, Ferguson (Cooper, 88'), Morison, Abdou. Subs not used: King, Martin, Thompson, Romeo.
Scorers: Gregory (6', 35'), Craig (26'), Hutchinson (84')
Bookings: Craig, Morison
Referee: D. Handley
Attendance: 11,750 (1,281)