A thrilling rugby tale of the tortoise and the hare as Leinster suffered yet more heartbreak at the hands of Ronan O'Gara and his La Rochelle giants.
The boys in blue played like men possessed in a sensational opening that looked to have put them on the way to that coveted fifth star on the jersey.
Three tries in the opening 12 minutes made it seem that they had one hand on the trophy.
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In the end, though, it was La Rochelle who joyously celebrated going back to back as, just metres away, Leinster's injured captain Johnny Sexton offered commiserations to his devastated colleagues.
This was not to be the glorious farewell that the province wanted for Sexton and Stuart Lancaster, not the revenge on Les Maritimes for killing their European dream in the 2021 semi-final and last season's final, not the home victory they have craved.
In Marseille, Leinster failed to score a try and it ultimately cost them dearly.
This time they scored three but it was still not enough as, slowly, surely but, in truth, inevitably as the second half went on, La Rochelle reeled them in. They finally took the lead seven minutes from time.
Leinster's regret will be compounded by the fact that they were just metres from La Rochelle's try line in the 78th minute when replacement prop Michael Ala'alatoa's clear-out was deemed dangerous and he was red carded.
La Rochelle were awarded a penalty and that was that, the chaotic end game summed up by the fact Leinster finished with 13, La Rochelle with 14.
Earlier, it seemed that Leinster had learned the big lessons from last year. They went wide whenever possible, they kicked in behind and La Rochelle's defence, so full of big men as it is, couldn't cope.
The visitors were immediately on the back foot when Brice Dulin sliced a clearance into touch.
Sheehan's throw was taken by the rampaging Jack Conan, who returned the compliment and the dynamic hooker was in for the first try with 40 seconds on the clock.
Ross Byrne converted, then James Lowe delivered a clutch 50:22 kick into the left corner.
Leinster's lineout maul was illegally disrupted but Byrne picked out Jimmy O'Brien on the right touchline and he dived home.
Byrne hit the far post with the tough conversion attempt - and did the same again when Sheehan got in for his second try.
Gibson-Park was at his best to see the opportunity after Tadhg Furlong pounced loose ball and Sheehan was there to score in the corner.
So 12 minutes gone and Leinster were 17-0 up. La Rochelle also scrum-half Tawera Kerr-Barlow in the sin bin.
But once they found some rhythm in attack, La Rochelle were in business. Jonathan Danty handed them a lifeline in the 19th minute.
The big centre bounced off Garry Ringrose, then out-paced Robbie Henshaw to score. Antoine Hastoy made it a 10-point game.
However O'Gara was soon slamming the desk in the coaches box in frustration as ref Jaco Peyper awarded Leinster a breakdown penalty and Byrne fired over for a 20-7 lead in the 24th minute.
It was Leinster's turn to be frustrated when ruck ball was knocked out of Gibson-Park's hands and, with Leinster flying into rucks, things got tasty on the half hour mark as rivals squared up.
The Blues also lost their skipper James Ryan to a HIA and he didn't return, which was a blow, but Byrne did tack on another penalty.
However it was La Rochelle who scored again before the break. Sheehan initially put Leinster in trouble with an attempted off-load but Keenan denied Pierre Boudehent a try.
But the men in yellow were not to be denied and, after making the hard yards on the right flank, this time it was easy for their other centre, UJ Seuteni, to race over unimpeded from Hastoy's pass and the latter made it 23-14 with his conversion.
It was the visitors who made the early running in the second half and Seuteni's break forced a penalty that Hastoy converted, but Byrne quickly replied from a scrum penalty.
Gibson-Park's misjudged clearance kick straight into touch was the start of a series of errors by Leinster players but they survived another scintillating Seuteni burst, with Keenan and Henshaw bringing him down.
Byrne had a kick blocked down, Lowe made two kicking errors but Leinster kept making big turnovers in their own half, and in their own 22 - Sheehan, van der Flier, Henshaw and his replacement Charlie Ngatai all contributed.
But still the big men came forward as the clock ticked down to into the last 10 minutes and there was a sense of inevitability about what came next.
Leinster conceded penalty after penalty as they tried to keep their rivals out but it was not to be and Georges Henri Colombe powered over.
Worse for the Blues, Rónan Kelleher was yellow carded before Hastoy kicked La Rochelle ahead.
Back came Leinster and there was a chink of light when Danty was sent to the bin for a high tackle on Caelan Doris.
Ringrose came close to the try line but was smashed and, soon after, Ala'alatoa's impetuous act brought down the curtain on Leinster's trophyless season.
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