Leinster must beware of the original kings
Leinster v Toulouse, Live BT Sport 2 (Tom, 1.0)
The journey to the top in Europe and, indeed, back to the top can usually be traced to that one magical match.
Take Leinster. The greatest under-achievers in the game from 2002 onwards, finally found the answers at Croke Park in 2009.
The Heineken Cup semi-final stumping of champion Munster broke the back of their hike to the summit of the game.
They were never going to let the opportunity to conquer Europe slip once they overcame the champions.
It made it even more meaningful when they completed the epic ride against two-time champions Leicester Tigers.
Last season, Jonathan Sexton was convinced Leinster had the tools to take care of the champions Saracens, on the hunt for three-in-a-row.
There was something of a sense of déjà vu about the way they dominated the holders in a signature performance in the quarter-final at The Aviva Stadium.
Once again, it would have been wasteful, in the extreme, were they not to have bowled over Scarlets and Racing 92 to capture their fourth crown.
The achievement elevated Leinster to a view shared by Toulouse, the original and only other four-time winners.
It had taken Leinster six years to regenerate from Ulster in Twickenham to Racing in Bilbao.
For Toulouse, the outpouring of emotion that greeted their 28-27 victory over Leinster at Stade Wallon in round two was a reflection of how much has changed in Europe.
Don't forget, the shoe was on the other foot when Denis Hickie's spectacular try was the enduring memory from that 41-35 quarter-final for the ages in 2006.
Back then, a rugby generation ago, champions Toulouse were the chased and Leinster the chasers.
However, Toulouse's grip on greatness has loosened, last winning the title in 2010.
It has taken them eight years to become a force to be reckoned with again
And yet, there was a sense of how former glories could be restored in the aftermath of that one day last October.
Toulouse have improved since then to head the Top-14 and head to Leinster intent on turning the dream of a fifth European Cup into a reality.
It is one thing to beat Leinster in France; something entirely different to do it in The RDS, a clash that could change everything again.