It has all come down to this, a hostile night in Paris where war will be waged as a matter of pride for one club at home -- and a matter of necessity for another to return home for the quarter-final.
Whose need is greater? Whose motivation is deeper? The answers will only be revealed after bodies have been spent and minds exhausted.
It only takes one sub-standard shift for a whole season and a way of playing the game to be called into question. This is why consistency is paramount.
Leinster cannot afford to stand back and play the role of passive tourists. They have to rock an immobile Racing Metro back onto their heels through tempo and cohesion.
When Racing turn, they do so with all the speed of an ocean-liner.
When Leinster get in behind the first line of defence, they can devour ground and assemble points quicker than all except Toulouse. These are the keys to the quarter-final kingdom, tempo and beating the first tackle.
From there, the offloading skills of all bar Mike Ross and Leo Cullen, the anchors of the ship, come into play.
It is tempting to suggest Leinster have reached that point in their development when they have to look after what they do with the ball and everything else will look after itself.
You could call it 'The Jamie Heaslip ethos'. He has the confidence, the belief that when he is at his best, it is good enough to make all the difference. Sadly, he will not play. Sean O'Brien resumes at number eight and Rhys Ruddock is dropped in ahead of Dominic Ryan on the flank.
Certainly, Luke Fitzgerald could do with that bombproof belief in his ability. He was way off the pace against Saracens, the internationally under-used England wing David Strettle blasting outside him three times.
The three-quarter must learn how to relax into a game. He is eager. Fine. He is passionate. Fine. He is over anxious. Not fine. Fitzgerald is forcing things that aren't there for the taking.
Similar to the old Gaelic games adage of 'take your points and the goals will come', do the basics well and the rest will follow on from that as the confidence returns to him.
He is a class act. Full stop.
This could be the perfect time for Fitzgerald to remind Declan Kidney of his attributes against what is, basically, a second-string Racing outfit, retaining just three players, all backs, from the side that shipped 38 points to Leinster in Round One.
Racing Metro: D Scarbrough; B Fall, V Vakatawa, A Masi, J Saubade; J-M Hernandez, N Durand (capt); J Brugnaut, G Arganese, S Zimmermann, S Dellape, F van der Merwe, R Vaquin, B le Roux, A Galindo.
Leinster: I Nacewa; S Horgan, B O'Driscoll, G D'Arcy, L Fitzgerald; J Sexton, I Boss; C Healy, R Strauss, M Ross, L Cullen (capt), N Hines, R Ruddock, S Jennings, S O'Brien.