Harlequins capitalised on a dire Racing Métro performance with an excellent 32-8 victory at the Stade de la Beaujoire in Nantes.
The 2012 Premiership champions powered the scoreboard and scored four tries through Nick Evans, Nick Easter, Charlie Walker and Karl Dickson.
Evans in particular was outstanding, manipulating the Racing defence and taking the opening try with aplomb as he showcased some vintage form.
Harlequins had slipped to losses in their first two matches in Pool 4 against Clermont Auvergne and the Scarlets, but their recent form in all competitions has been on the rise and this triumph in Nantes threw them back into contention in Europe.
It was that renewed confidence that backed Harlequins early intent to play into the corner from a penalty and control matters with their line-out - but the best that came from it was a cutting break by Danny Care off the base of the scrum before his offload unfortunately landed in the hands of the giant Jone Qovu.
The opening quarter was dictated by a battle for control between Benjamin Dambielle and Evans, before the former All Black fly-half scorched through a gap in Racing's defence to step Eddy Ben Arous for the opening try.
The visitors' scrum outmuscled Racing's pack early on to yield a first penalty opportunity, with the Evans converting from out wide for a precious 10-0 lead to Harlequins with 23 minutes gone.
Racing desperately needed a response but their composure was sorely lacking - a combination of a cruel bounce and slip from Juan Martin Hernandez resulting in Harlequins attacking from a line-out deep in home territory, but they came away with nothing to show for it after Matt Hooper's attempted score was ruled out by the TMO.
Harlequins though were not made to wait long for more points. Former England number eight Nick Easter thundered over by spinning out of a tackle to go over near the posts, leaving Evans with a simple conversion.
Racing's defence was abject throughout as their English guests enjoyed ample space around the fringes to execute their offloading game, Care in particular revelling on the open ground.
Belatedly threatening as time ticked away before the interval, sniping breaks from Marc Andreu meant Racing at least gave the impression of a fightback as Dambielle struck a long-range penalty. The Parisians though faced an uphill battle at 17-3 down.
Three-time Heineken Cup winner Jonathan Sexton was introduced off the bench for Racing but the momentum was firmly in Harlequins' favour as Hernandez faltered again.
The full-back appallingly muffed Care's box kick and young winger Charlie Walker wasted no time to scoop up the loose ball and then scamper away down the right touchline for the visitors' third try.
It was a moment of pure carelessness from the Argentina international that aptly summarised Racing's night, even with Sexton and new signing Mike Phillips in tow off the bench.
Buoyed by a frustrated crowd Racing did eventually roar back, France international Bernard Le Roux at the tail end of a rolling maul to at least limit the gap to 19 points.
Indiscipline crept into Racing's downfall as they struggled to compete at the breakdown and then contain Harlequins' driving maul.
It was no surprise to see one of those, with Karl Dickson on the end of it, bring about a bonus point try and underline an outstanding effort from Harlequins.
The missed conversion from Evans was his only wayward strike of the night. Harlequins did little wrong in general, whereas Racing's list of inconsistencies and flaws - beginning with questions surrounding their attitude and defence - will take some time to repair.
For Racing Métro:
Try: Le Roux
Tries: Evans, Easter, Walker, Dickson
Cons: Evans 3
Pens: Evans 2
Racing Métro: 15 Juan Martin Hernandez, 14 Benjamin Fall, 13 Fabrice Estebanez, 12 Alexandre Dumoulin, 11 Marc Andreu, 10 Benjamin Dambielle, 9 Maxime Machenaud, 8 Jone Qovu Nailiko, 7 Bernard Le Roux, 6 Wenceslas Lauret, 5 Juandre Kruger, 4 Francois van der Merwe, 3 Luc Ducalcon, 2 Dimitri Szarzewski (capt), 1 Eddy Ben Arous.
Replacements: 16 Virgile Lacombe, 17 Davit Khinchagishvili, 18 Brian Mujati, 19 Karim Ghezal, 20 Camille Gerondeau, 21 Mike Phillips, 22 Jonny Sexton, 23 Juan Imhoff.
Harlequins: 15 Mike Brown, 14 Tom Williams, 13 Matt Hopper, 12 Tom Casson, 11 Sam Smith, 10 Nick Evans, 9 Danny Care, 8 Nick Easter, 7 Chris Robshaw (capt), 6 Luke Wallace, 5 George Robson, 4 Charlie Matthews, 3 Will Collier, 2 Dave Ward, 1 Joe Marler.
Replacements: 16 Harry Allen, 17 Mark Lambert, 18 Paul Doran-Jones, 19 Nick Kennedy, 20 Joe Trayfoot, 21 Karl Dickson, 22 Ben Botica, 23 Charlie Walker.
Referee: Leighton Hodges (Wales)
Assistant referees: Gwyn Morris, Simon Rees (both Wales)
TMO: Gareth Simmonds (Wales)
by Ben Coles