It was an afternoon of joy and then utter despair as Danny Senda scored his first goal for Millwall but was then stretchered off with a dislocated knee-cap as The Lions wrapped up their League One campaign at the County Ground.
Senda, operating in an unaccustomed central defensive role for the first time this season, was perfectly placed to head Kenny Jackett's side ahead after just six minutes.
But a 30th-minute penalty from Simon Cox (30mins) coupled with an Anthony McNamee effort on 50 minutes secured victory for Swindon.
Any thoughts that this was going to be an end-of-season kickabout were quickly dispelled as both sets of players seemed intent to sign off the campaign on a positive note.
From the visitors' point of view, no fewer than six of the starting XI were either out of contract in the summer, or in the case of Jem Karacan and Jay Simpson, returning to Reading and Arsenal following their loan spells at The Den.
It took just two minutes for Millwall to ask questions of the home defence when Ahmet Brkovic floated in a free-kick from the left, Marc Laird on to Paul Robinson, but the ball skimmed off his shin and flew wide.
Four minutes later another Brkovic delivery picked out Senda, who ghosted in behind a static Robins rearguard to head home from five yards for his first goal since 1st January 2005.
However The Lions nearly threw away their early lead within a couple of minutes when the ball was deflected into the path of Billy Paynter, but Marcus Bignot was on hand to make a magnificently timed clearance and hook the striker's goalbound shot off the line.
Former Swindon keeper Rhys Evans came to the rescue after 20 minutes with a flying save to keep Michael Timlin's header out, while Karacan's 25-yard screamer flew narrowly over as the two teams traded scoring chances.
The home side were handed a huge stroke of luck on the half-hour mark when referee Haywood penalised Robinson for a foul on Paynter, and up stepped Cox to smash the spot-kick into the bottom left corner for the leveller.
Millwall's response was immediate, and it took a fine stop from Peter Brezovan in the home goal to deny Laird's 25-yard thunderbolt after 32 minutes.
The Lions had as blatant a penalty as you could wish to see denied them five minutes before the break, with a baffling piece of officiating.
Bas Savage picked up the ball on the left, was clearly impeded but the referee waved 'play on'. Savage did, was then sent flying in the area following another clumsy challenge, but could only look on in utter disbelief as the referee Haywood brought play back to where the original foul had taken place.
Even then, Millwall could have taken advantage when Brkovic - who produced some excellent dead ball deliveries throughout - found Neil Harris, but his header glanced a yard over.
Another Brkovic delivery then picked out Senda who, for the second time in the opening half, stole in unmarked. On this occasion the defender's first-time volley flew wide.
That proved to be Senda's last contribution to the game because in Millwall's next attack, he seemed to twist awkwardly in the box and after attention from the medical team, was stretchered off in the recovery position with his anguish clearly visible.
Richard Shaw stepped into the fray, but within five minutes of the second half he could do little to prevent a rasping drive from McNamee hitting him and ricocheting into the back of the net with keeper Evans stranded.
Insetad of accepting their lot, The Lions did what they have done on so many occasions this season - battered teams with possession, created chances but been unable to convert their superiority into goals.
There were plenty of opportunities to haul themselves level in the final half an hour with Ryan Smith squaring for fellow sub Ali Fuseini 18 yards out, only for the latter to shoot straight at keeper Brezovan.
Smith himself then cut in before firing wildly off target with a right-foot effort whilst Robinson headed another Brkovic free-kick over.
With seven minutes remaining Millwall looked to have grabbed their second goal when Smith took advantage of a slip by Patrick Kanyuka and dinked in a cross for Harris, but his point-blank header was brilliantly parried by Brezovan before the keeper grabbed the ball at the second attempt.
Skipper Robinson was left holding his head in his hands after heading wide of an open goal from a Brkovic free-kick two minutes from time before Laird drilled over from long range as The Lions kept pressing forward.
Simpson nearly signed off from his season-long loan in spectacular fashion when he sent two defenders the wrong way with a mazy run deep into injury-time, but having done all the hard work he dragged his shot agonisingly wide with Millwall's last chance of the game.

Swindon Town: Brezovan, Vincent, Easton, McGovern, Aljofree, Timlin, Paynter (Joyce, 73mins), Allen (Kanyuka, 62mins) , Morrison, McNamee, Cox. Subs not used: Smith, Hammonds, Thompson.
Booking: Kanyuka (87mins)
Scorers: Cox (30mins, pen), McNamee (51mins)

Millwall: Evans, Senda (Shaw, 45mins), Bignot, Laird, Robinson, Bakayogo, Simpson, Karacan (Fuseini, 68mins), Harris, Savage (Smith, 54mins), Brkovic. Subs not used: Edwards, Spiller.
Scorer: Senda (6mins)

Ref: M. Haywood
Att: 7,781