Ronan O'Gara v Johnny Sexton.
It's hard to think of something like it in modern Irish sport. Two of the finest players of Irish rugby's "golden generation" - not only in their position of out half, but pitch-wide.
This was the classic "passing of the guard" narrative, but with so much more added on, and it took on yet another edge with their reported clash at half-time of Saturday's Champions Cup final in Dublin.
We've boiled down the saga of Ronan O'Gara and Johnny Sexton to five crucial flashpoints from the past 15 years. From their clashes in O'Gara's playing days for Munster, to sharing the pitch with Ireland, to an unexpected team-up in O'Gara's early coaching days, and on to Saturday's soon-to-be-infamous meeting in Dublin, these are the five incidents that have defined a classic rivalry of Irish sporting history.
O'Gara - Sexton: A true classic of modern Irish sports
Chapter one: That game in Croke Park and the emergence of a star
When the Heineken Cup semi-final came to Croke Park in 2009, it was undoubtedly the biggest club game in Irish rugby history hitherto. One could argue that, bar perhaps the 2012 final which saw Leinster meet Ulster, nothing has surpassed it since.
Munster came into the game as defending champions, having won two of the previous Heineken Cup titles in 2006 and 2008. Leinster, on the other hand, were still chasing their first European crown. Hard to picture now, right?
At the centre of Munster's successes in the mid-2000s was out-half Ronan O'Gara, who also entered the 2009 semi-final as the hero of Ireland's recent Grand Slam triumph.
And then, an injury to Leinster out-half Felipe Contepomi early in the game gave rise to a new star of Irish rugby.
This was the third year of Johnny Sexton at Leinster, but it was the first time he had been given a major opportunity in a big game. In front of a sold out 82,000+ crowd at Croke Park, Sexton and Leinster ran riot.
The centrepiece of Leinster's 25-6 win was Brian O'Driscoll's breakaway try - as the centre intercepted a wayward pass from O'Gara on the halfway line and never looked back.
It was after a try from the other Leinster centre Gordon D'Arcy, however, that the defining image of this rivalry got an early release.
With Leinster celebrating, the pitchside cameras caught 23-year-old Sexton yelling in the face of his Munster counterpart O'Gara, birthing a competition between the two for supremacy at number ten for Ireland.
Chapter two: South Africa '09 and a changing of the guard
Leinster would go on to claim the 2009 Heineken Cup crown, with Sexton starting the final in Edinburgh and kicking 11 points in their 19-16 victory over Leicester Tigers.
After a disappointing outing for O'Gara on that summer's Lions Tour of South Africa, the debate was rife ahead of the autumn internationals as to who would start at out-half for Declan Kidney's Ireland team against the Boks.
O'Gara started in a 20-20 draw with Australia and was flawless from the kicking tee. But, when world champions South Africa came to town a fortnight later, it was Sexton who started at ten.
Five Sexton penalties later, and Ireland ran out 15-10 victors at a foggy Croke Park, claiming a famous victory against the 2007 World Cup winners.
Though O'Gara would briefly win the jersey back in the later stages of the 2011 Rugby World Cup, the South Africa game could be looked back on as the moment the status of power formally shifted.
O'Gara retired in 2013 out of the Ireland squad entirely, with Sexton established as the first choice out half for the years ahead. At the end of the club season, Sexton departed Leinster in a surprising move, to join Racing 92 in the Top 14...
Chapter three: Racing 92 and the pair are reunited in Paris
Shortly after Sexton's departure to Paris was announced came some stunning news. The new appointment at Racing, and Sexton's new defensive coach would be...Ronan O'Gara.
After nigh-on four years competing for the Irish out half slot, and clashing on the pitch with their respective provinces, O'Gara and Sexton would now be on the same side.
It was an unexpected crossover in the French capital - but Sexton assured the Irish media that he was delighted to get the opportunity to continue learning from O'Gara, eight years his senior.
There was a disappointing lack of silverware during Sexton's two years at Racing, and he would return to Leinster at the end of the 2014-15 season.
One year later, the club would finish runners up in the Champions Cup and claim the Top 14, with O'Gara still a member of the coaching staff. Midway through the 2017-18 season, O'Gara departed for New Zealand, enjoying two hugely successful years as assistant backs coach at the Crusaders, which brought two Super Rugby titles. Then, he made his move back to Europe as head coach of La Rochelle...
Chapter four: Marseille 2022 and they meet again
O'Gara and Sexton both enjoyed success in the late 2010s, with Sexton's return to Leinster bringing a fourth Champions Cup medal in 2018, in a year in which he was crowned World Rugby player of the year for his efforts in helping Leinster to that crown, and Ireland to another Grand Slam and a first victory over the All Blacks on home soil.
When ROG arrived in La Rochelle, the club had never won a major honour, but he led them to the 2021 Top 14 and Champions Cup finals, before the greatest achievement yet came in 2022 - at Sexton's expense.
In a packed out - and predominantly yellow - Stade Vélodrome in Marseille, O'Gara led his La Rochelle team to an historic Champions Cup title, with a dramatic last-gasp try from Arthur Retiere snatching victory from the jaws of defeat.
It was a stunning victory for La Rochelle and their still up-and-coming head coach - but almost an equally stunning defeat for Leinster.
Sexton and his teammates were shellshocked at full-time but, with the following year's final set for Dublin, there was a determination for them to put things right on home soil.
Chapter five: Dublin 2023 and the La Rochelle dynasty begins
Incredibly, the 2023 Champions Cup final saw Leinster meet La Rochelle for the second consecutive year - the first time in history that back-to-back finals had seen the same two teams compete.
It also meant yet another meeting of Ronan O'Gara and Johnny Sexton. The coach v player dynamic was the fourth different dynamic of their rivalry - they had been competitors on the pitch, international teammates, part of the same team, and now faced down once again with Sexton and Leinster out for revenge.
Only, Sexton was sidelined for the game through an injury sustained in the final game of yet another Irish Grand Slam victory earlier this year.
What transpired on the pitch was extraordinary. Despite a lightning start putting Leinster 17-0 up inside the first 11 minutes, La Rochelle held their nerve, and worked their way back into the game, before a late try from Georges-Henri Colombe gave the French side a scarcely believable second Champions Cup in a row, and O'Gara the credentials to place him in the echelon of world rugby's elite coaches in 2023.
And, despite Sexton not being on the field of play, there was yet another chapter which reignited the "rivalry" aspect of this great head-to-head.
Half-time saw La Rochelle go in nine points down, but O'Gara was reportedly unhappy with some of Jaco Peyper's refereeing in the first-half (hat tip to Ruaidhri O'Connor on the Second Captains Podcast for the information here), and made a beeline for the referee's room in the tunnel. He was met with Leinster legend Sean O'Brien, alongside Sexton.
The clash was reported in the aftermath of the game, with EPCR noting that they would investigate the incident - but by that stage, O'Gara and La Rochelle had their celebrations well underway, and Leinster were left to reckon with how on Earth they had let this one go.
For O'Gara, whose Ireland career ended in such disappointing and heartbreaking circumstances when he was denied the chance to give the Ireland fans at Lansdowne Road a proper farewell, this victory at that very ground may have been the sweetest yet.
O'Gara v Sexton. It has been a defining storyline of Irish rugby ever since that fateful spring day in 2009, and the action of the last two years has only given it a new lease of life.
Two of the greatest Irish players of all-time on the pitch, their rivalry has now moved to a new dimension, with O'Gara flourishing beyond expectations in his coaching career - almost at the direct expense of his old rival.
Long may this great (and generally good-spirited) rivalry continue.