Stefan Bailey was sent off in shame and Zesh Rehman scored a calamitous own goal as QPR gifted Luton a lucrative televised FA Cup fourth-round clash with Blackburn.
Bailey saw a straight red card for a horrific lunge on Ahmet Brkovic - who had to be carried off on a stretcher - sparking an unseemly 20-man shoving match.
Referee Michael Jones was left holding his nose after the midfielder and Damion Stewart contested the decision, although any intent from the Rangers players appeared unlikely.
Rehman then turned the ball past his own keeper with 10 minutes remaining to hand Mike Newell a reunion with his former club on Saturday.
Both sides were forced to make changes for the rearranged replay, with Luton minus both their goalscorers from the original 2-2 draw at Loftus Road following Rowan Vine's move to Birmingham and an injury to Warren Feeney.
John Gregory, a cup finalist with Rangers in 1982, welcomed back highly-rated winger Lee Cook from suspension and recalled Marc Nygaard alongside Dexter Blackstock in attack.
After a scrappy opening Luton failed to deal with Jimmy Smith's corner but Stewart could not get the ball out from under his feet and the danger was cleared.
Nygaard then headed a Smith free-kick over the crossbar before Luton created their first decent chance, Adam Boyd's through ball catching QPR square but the onrushing David Bell's angled shot flying narrowly wide.
Rangers immediately hit back and Blackstock's cross just evaded Nygaard with the home defence suddenly looking like strangers.
Town keeper Dean Brill, just recalled from a loan spell at Gillingham, then kept the scores level with a superb double save, diving low to his right to parry Smith's 20-yard effort and recovering in time to deny Blackstock from the rebound.
Bell had the ball in the Rangers net shortly before the break but was clearly offside, while Blackstock wastefully nodded Mauro Milanese's cross wide.
Rangers began the second half with more purpose and Nygaard combined well with Smith down the right before grazing the crossbar with a shot from an impossible angle, although the pull-back to Cook looked to be the more profitable option.
Giant centre-half Stewart then met Smith's inswinging corner but thumped his powerful header inches wide with Brill motionless.
Nygaard missed a gilt-edged chance to open the scoring 20 minutes from time following another superb run from Cook.
The tricky winger glided jinked past a succession of defenders before his reverse pass put Nygaard through, but the striker smashed the ball into the side-netting from eight yards out.
The game exploded in the 74th minute when Bailey went in two-footed to contest a 50-50 ball with Brkovic.
After an ugly melee was dispersed, Bailey was dismissed while Brkovic departed in some discomfort on a stretcher.
And six minutes later Rehman inadvertently settled the affair when he turned substitute Dean Morgan's cross-shot past the helpless Royce.
Morgan should have grabbed a second when he burst clear during five minutes of stoppage time, but he blasted his shot over the top.
Teams
Luton Brill, Foley, Davis, Coyne, Perrett, Bell, Robinson,Langley, Emanuel, Boyd, Brkovic (Morgan 76).
Subs Not Used: Barrett, O'Leary, Keane, Stevens.
Booked: Coyne, Morgan.
Goals: Rehman 80 og.
QPR Royce, Rehman, Mancienne, Stewart, Milanese,Smith (Ainsworth 84), Bailey, Lomas, Cook, Blackstock,Nygaard (Ray Jones 72).
Subs Not Used: Cole, Kanyuka, Furlong.
Sent Off: Bailey (74).
Att: 7,494
Ref: M Jones (Mansfield).