Late on Saturday in a room beneath the Stade de France, once the television lights had been turned off and French interviewers were looking elsewhere, Max Guazzini, media magnate and owner of Stade Français, nodded his congratulations towards Dean Richards, director of rugby at Harlequins. Not many people spoil a Guazzini party and the awkwardness of the moment showed.

A day that had been scripted, from start to firework-display finish, as another Stade win in Paris ended with a strained smile. Last season Stade, one of the benchmark Heineken clubs, bundled Harlequins out of the cup during the pool stages; this year they themselves will be out before Christmas if they do not win in London on Saturday. As Guazzini said: "We have one chance, but that is the only chance."

Before Saturday Stade had not lost in 17 games in Paris and only once in 32 since 1998. This defeat, by a young side with no European history to speak of and only a brief time back in the Premiership, may not signal the shifting of tectonic plates but it sent a seismic shudder through the Heineken bedrock. The manner of victory suggested they will be a thorn in anyone's side come the knockout stages.

It was not pretty, more a smash and grab raid. Harlequins jumped on Stade's mistakes in the first half to go 12 points up in 25 minutes and battened down the hatches in the second half when the home side threw everything at them. "We warned the boys as to what to expect," said Richards, who sampled some pretty tasty Paris days as a player with both England and Leicester. "The boys stood their ground and didn't give an inch, and we will have to do exactly the same next week. We have to back it up with a win at the Stoop. They will come back, they will feel wounded and will have all their guns blazing."

Blazing guns was about all that Guazzini did not organise for Saturday after moving the game from Stade's home, Stade Jean-Bouin (capacity 12,000) across town and into the 80,000-seat national stadium. The hour-long pre-match floor show included girls from the Moulin Rouge, jousting knights, a medieval parade and a couple of eagles who were rather more sure-footed than much of the early play.

For five minutes it appeared as though Harlequins were the ones with stage fright but the Stade backs upstaged them time and again. When Danny Care chipped into the home 22 , Djibril Camara and Julien Saubade got into a fearful tangle, the ball ricocheting around until Tom Williams scooped it up to put Harlequins ahead.

Saubade was banished for his sins - replaced by Mark Gasnier, a rugby league import from Australia who somehow finished up running the Stade show in the second half when the talented Juan Martín Hernández was shifted to full-back. Before then, however, there was time for one more costly first-half howler.

Camara dithered in collecting Mike Brown's hopeful hoof upfield and was splattered by the Quins second-row James Percival. Once again the ball ran free and this time Jordan Turner-Hall got in first, the centre up and gone before Stade had time to think about organising a chase.

But that was about the extent of Stade's largesse. Three minutes from the interval Care was sin-binned for conceding three points when five or seven looked on and before the England scrum-half was back in the action, Stade had reduced the lead to two points.

From then until the whistle, Harlequins were marvellous in defence. Led by their captain, Will Skinner, who tackled himself to the point of exhaustion, and supported by Care and Nick Easter, Quins stopped Stade again and again, and even earned a Nick Evans, until with victory seemingly in the bag, there was one final indiscretion. With nothing left on the clock and a penalty won, Care spoke to referee Alan Lewis - "I may have said something," the scrum-half said later - and the penalty was reversed, condemning Harlequins to another minute under siege.

"It is not just the defence we had but also the discipline," said Richards, almost hitting the nail on the head. "A lot of the boys grew up here." As for Guazzini, the part-time showman will have to be satisfied with drawing 77,000 - the highest attendance for a Heineken pool game.

Stade Français Camara; Arias (Bousses, 73), Bastareaud, Liebenberg, Saubade (Gasnier, 20); Hernández, Oelschig (Albouy, 74); Roncero, Szarzewski (Blin, 56), Marconnet, Vigouroux, Taylor (Mauro Bergamasco, 56), Parisse (capt), Rabadan, Leguizamon.

Try Leguizamon. Con Hernández. Pen Hernández.

Harlequins Brown; Williams, Tiesi, Turner-Hall, Monye; Evans, Care; Jones, Fuga, Ross (Robson, 63), Percival, Evans, Robshaw, Skinner (capt), Easter.

Tries Williams, Turner-Hall. Con Evans. Pen Evans.

Sin-bin Care, 37

Referee Alan Lewis (Ireland).

Attendance 76,569.