Lee Gregory bagged his first ever hat-trick for Millwall - but Derby Jeff Hendrick left The Lions clinging onto their Championship status with an agonising late leveller in a six-goal Den thriller.
Gregory struck twice in the opening half - including a penalty - to put Neil Harris charges 2-0 ahead. Tom Ince reduced the arrears three minutes before the break and although Gregory restored Millwall's two-goal cushion on 50 minutes with his second spot-kick of the game, The Rams fought back. Two goals in the last 20 minutes courtesy of sub Chris Martin's 70th-minute penalty and Hendrick's 85th-minute cracker ensured that the points were shared.
For Millwall this was very much a case of two points dropped, and a win for Rotherham against Reading on Tuesday night will condemn The Lions to League One football.
However whatever the outcome of that game, Harris' Lions could hold their heads up after giving their absolute all against a Rams side pushing for a top six place.
For long spells of this season there has been little for Millwall fans to cheer about, but the appointment of Harris as interim manager in March has seen the purpose, passion and pride restored.
Millwall's starting XI showed three changes to the team that played at Blackburn in midweek. Defender Mark Beevers was sidelined through illness, with Jos Hooiveld stepping back in to partner Sid Nelson in the heart of the back four whilst the two Lees - Martin and Gregory - replaced Magaye Gueye and Ricardo Fuller, both of whom dropped to the bench. Young
Derby were forced to re-shuffle their playing pack with a trio of changes. Darren Bent was ruled out due to illness so Conor Sammon, just back from a loan spell at Rotherham United, made his first start for The Rams since May 2014. Craig Forsyth and Simon Dawkins also returned in the team.
The outstanding Aiden O'Brien, who seems to just get better with every game, had a chance in the opening quarter of an hour when he ran at the Derby defence but fired a tame shot straight at goalkeeper Lee Grant despite having Gregory and Jimmy Abdou as options on either flank.
Two minutes later the visitors went close when a long throw by Ryan Shotton found Richard Keogh, but the captain's header drifted inches wide.
Millwall kept pressing with O'Brien testing Grant in the visiting goal from the edge of the box, whuilst the young striker had the ball nicked off his toes as he was about to pull the trigger following a superb pass from Nicky Bailey.
The Lions' pressure finally told in the 26th-minute when Gregory capitalised on a poor clearance from Grant that reached the edge of the box and although his angled drive was heading goalwards, Craig Forsyth had an opportunity to clear but seemed to slice the ball backwards and into his own net.
There was no doubting the scorer of Millwall's second, however, when Gregory clinically despatched a penalty beyond Grant's reach after Martin's cross from an O'Brien flick-on was handled by Keogh.
Roared on by the home fans, Millwall had the bit between their teeth and sent over a succession of teasing crosses that Derby did well to defend.
However the visitors were handed a lifeline three minutes before half-time when Sid Nelson gave away a cheap freekick 20 yards out with a tug of Conor Sammon's shirt. Up stepped Ince - of often Millwall's nemesis - to curl a shot beyond the reach of David Forde and into the bottom left corner.
Millwall - and Gregory - took just five minutes of the second-half to restore that two-goal lead. This time Hendrick was penalised for holding Dunne as Martin sent over a corner and referee David Coote pointed to the spot again. Gregory made no mistake for a second time, securing his first ever Football League hat-trick.
The former AFC Halifax frontman, like strike partner O'Brien, worked himself into the ground and his hat-trick was a just reward for his effort.
Derby brought on top scorer Martin for Dawkins and within six minutes of his introduction, the sub had pulled a goal back with the third penalty of the game. This time, O'Brien's soft challenge on Ince as he darted into the box darted into the box saw ref Coote point to the spot. Martin stepped up and halved the deficit to ensure a nervy final 20 minutes.
The Rams had keeper Grant to thank for producing two stunning reflex saves, firstly to tip over O'Brien's 73rd-minute curler and then a header from Martyn Woolford just three minutes later as Millwall pushed forward again.
But it was Derby who grabbed a late point with the goal of the game - Ince teed up Hendrick and he in turn hit a net-busting volley from the edge of the area to level up at 3-3.
Millwall had a half-chance to nick all three points deep into injury-time when sub Ricardo Fuller, a 78th-minute replacement for hat-trick hero Gregory, used his power and strength to work a path towards goal, but a perfectly timed tackle from Keogh averted the danger as he prepared to shoot.
The Lions rightly left the field to a standing ovation and regardless of the final position in the League table on 2nd May, the 'P' factor - pride, passion and purpose - is back.
Millwall: Forde, Dunne, Harding, Abdou, Nelson, Hooiveld, Martin (Gueye, 90mins), Bailey, Gregory (Fuller, 78mins), O'Brien (Cowan-Hall 90+2mins), Woolford. Subs not used: Archer, Powell, Thompson, Taylor-Fletcher. Bookings:
Abdou (38mins), Dunne (50mins)
Gregory (26mins, 36mins pen, 50mins pen)
Derby County: Grant, Shotton, Keogh, Forsyth, Warnock (Albentosa, 64mins), Hendrick, Dawkins (Martin, 64mins), Hughes, Ince, Lingard (Russell, 46mins), Sammon. Subs not used: Roos, Christie, L. Naylor, Ward.
Bookings: Hendrick (48mins), Russell (53mins)
Scorer: Ince (42mins), Martin (70mins), Hendrick (85mins)
Ref: D Coote
12,571 (away 2,034)