A highly contentious first-half penalty by Ian Thomas-Moore coupled with a spectacular strike from Andy Robinson handed Tranmere Rovers a League One lifeline - but Leeds United's defeat at Charlton keeps the door for automatic promotion very much open for Millwall.
Victory for the Yorkshire outfit at The Valley coupled with The Lions defeat at Prenton Park would have seen Simon Grayson's charges return to the Championship.
But incredibly with just one game to go, all five teams beneath leaders Norwich City can still mathematically claim second spot - setting up an unbelievable finale on Saturday 8th May.
The Lions were not at their best in this match, especially in the second half, but a poor decision deep into first-half injury-time tipped the balance Tranmere's way, and they never looked back.
Rovers, who had been on the back-foot for large chunks of the opening half, took the lead with the softest of penalties.
Thomas-Moore and Paul Robinson went shoulder to shoulder in the area before the Rovers skipper turned past his Millwall counterpart and threw himself to the ground.
It was a dive Tom Daley would have been proud of, but instead of rightly booking the Tranmere striker, referee Neil Swarbrick inexplicably pointed to the spot.
To add insult to injury, it was Thomas-Moore who stepped up and slammed home the resulting penalty.
It was Thomas-Moore ironically, who had the first chance of the game on two minutes, heading over from close-range after the unmarked Ash Taylor had picked him out from a John Welsh free-kick.
A neat move at the other end saw Andy Frampton slip the ball through to Scott Barron on the left. He in turn cut in and saw his fierce shot blocked for the first corner of the game.
Liam Trotter then wriggled through to the by-line before his angled shot was blocked for another corner.
Steve Morison drilled the ball across the face of the goal and then Neil Harris' overhead kick flashed narrowly over as The Lions continued to prise and probe for the all-important first goal.
Harris had a 36th-minute free-kicked blocked by a sea of Rovers' defenders and Barron lashed the rebound wide, with Chris Hackett and a Steve Morison shot on the turn also off target as Millwall kept the pressure on.
It semed only a matter of time before a goal would be scored - but few amongst the home crowd could believe their luck when Thomas-Moore's theatrics were rewarded with a penalty, and the game's opener in the second minute of injury-time.
The Lions trudged off the field at half-time in a state of shock, and that still seemed to be the case in the opening exchanges after the break as Kenny Jackett's men were up against a rejuvenated Rovers.
On came Lewis Grabban for Barron; Grabban took his place on the right side with Hackett switching to the left flank.
And it was Grabban who was a whisker away from heading The Lions level within three minutes of entering the fray, with his effort from a Hackett corner dropping just wide of the post.
It took a smart save by Forde on the line to keep out Joss Labadie's goalbound header after Bas Savage had delivered an inviting cross from the right.
Gary Alexander was then introduced for Morison, but the striker barely had time to take up his position as Tranmere's Robinson, - on loan from Leeds - struck a stunning 30-yarder that gave Forde no chance in the visiting goal.
Robinson was denied a second goal just seconds later, with keeper Forde pushing his 25-yard free-kick round the post at full stretch.
Millwall tried their best to get back into the game and as news filtered through that Charlton had taken the lead against Leeds, the minutes seemed to tick away all the quicker.
Grabban had an effort diverted at full stretch by Peter Gulacsi whilst Harris and then Trotter both appeared to be impeded in the penalty area, but were cut no slack by the officials.
That merely heightened the frustration as The Lions suffered a second successive away defeat - and only third loss in 22 games.
The final day of the season will not be for the faint-hearted but one thing is sure - Millwall will need to be at the top of their game, and the supporters at their very loudest, if the team are going to beat Swindon.
The rest is out of our hands, but with the pressure of the highest order on Leeds United.
This time next week we will all know who has the strongest nerve and biggest hearts - bring it on!
Tranmere Rovers: Gulacsi, Logan, Goodison, Savage, Broomes, Thomas-Moore, Welsh, Taylor, A.Robinson (Curran, 81mins), Bakayogo, Labadie (Edds, 90+2mins). Subs not used: Cresswell, McCready, Barnett, Coughlin, Shuker.
Bookings: Broomes (36mins), Thomas-Moore (45+2mins), Robinson (74mins)
Scorers: Thomas-Moore (45+2mins, pen), Robinson (66mins)
Millwall: Forde, Smith, Frampton, Abdou, P.Robinson, Ward, Barron (Grabban, 52mins), Trotter, Harris, Morison (Obika, 75mins), Hackett (Alexander, 64mins). Subs not used: Sullivan, Grimes, Fuseini, Laird.
Bookings: Robinson (25mins), Trotter (65mins), Alexander (90+1mins)
Ref: Neil Swarbrick
Att - (away 2,013)