Birmingham City striker Marlon King hit a hat-trick to complete a remarkable turnaround as Lee Clark's men fought back from a 3-0 deficit to take a point from a ding-dong Den encounter.
King, who hit a treble for The Blues in their 6-0 win at The Den last season, scored after 45, 47 and 63 minutes to cancel out goals from Chris Wood (11 and 18mins) and Chris Taylor (13mins).
Lions boss Kenny Jackett will be wondering just how his team contrived to throw away a three-goal cushion having raced into such a commanding lead within the first 18 minutes, and of greater concern, that his side have yet to keep a clean sheet this season.
"Our goals against column is not good enough this season," said Kenny. "We've conceded three tonight and sides don't need many shots to score against us. You have to sustain periods of pressure where people have the ball and you've got to be able to soak it up. For us it's one shot, one goal - the defending wasn't good enough."
The Lions made one change from the team that drew at Crystal Palace on Saturday with Liam Feeney replacing Scott Malone, who dropped to the bench.
Birmingham created the first chance of the game but King's effort curled narrowly over the bar from 20 yards.
However it was Millwall who broke the deadlock on 11 minutes when Feeney launched a huge throw into the danger zone, Darius Henderson flicked on and Wood was on hand to nod the ball under the body of Jack Butland and into the back of the net.
Barely 120 seconds later The Lions had doubled their tally courtesy of a close-range finish from Taylor - his first senior goal for the club - with Butland looking slow to react again.
And it looked to be game over for Birmingham after 18 minutes when Liam Trotter's cross into the area took a slight deflection on its way to Wood, who won the initial header and then lifted the ball over keeper Butland.
Wood squandered a couple of glorious opportunities to clinch his hat-trick, first firing wide from eight yards after Trotter had played him in on goal and then rifling in an angled drive that Butland did well to push away at full stretch.
Millwall switched off defensively, crucially, deep into first-half injury-time enabling Leroy Lita to pick out King at the far post, and the former Gillingham striker made no mistake from three yards.
The Lions made a change at the break with Jack Smith replaced by Shane Lowry at left-back, and Butland was called into action within seconds of the restart with an excellent stop to prevent Henderson's rasping shot making it 4-1.
Instead Jackett's men found their lead further eroded when King picked up a low pass from David Murphy, turned and fired the ball past Forde to breathe fresh belief into the Birmingham ranks.
Stung by King's double strike either side of the break, the shell-shocked Lions tried to respond with Danny Shittu's effort blocked and then Henderson planting a firm header from Feeney's deep delivery straight into the hands of Butland.
But it was Birmingham who struck again, this time King taking advantage of the time and space afforded to him by smashing home in clinical fashion.
It could have been even worse for the shell-shocked Lions as Nathan Redmond forced a smart stop from David Forde in the home goal.
Millwall were reeling and Birmingham were rocking now as Lee Clark's men looked in danger of doing the inconceivable - adding a fourth goal.
Den boss Jackett responded by bringing on Andy Keogh for the tiring Feeney and it took a perfectly timed block by Ravel Morrison to deny the sub.
Another Keogh effort was deflected for a corner as The Lions pushed on in the dying minutes, but were so nearly caught out on the break when that man King dragged an effort narrowly wide following a quick counter-attack.
Henderson curled a free-kick a whisker wide in the final minute of normal time and despite a further four minutes of stoppage time that produced a flurry of three corners for Millwall, Jackett's men could not find that crucial fourth goal to win a game that had earlier been theirs for the taking.
Millwall:Forde, Dunne, Smith (Lowry, 46mins), Abdou, Shittu, Beevers, Feeney (Keogh, 66mins), Trotter, Henderson, Wood, C.Taylor. Subs not used: M.Taylor, Osborne, N'Guessan, Malone, Wright.
Birmingham City:Butland, Murphy, Caldwell, Davies, Mullins, King, Morrison, Elliott, Redmond (Burke), Lita, Robinson. Subs not used: Ambrose, Fahey, Packwood, Hancox, Diop, Doyle.
Att:9,258 (away 510)