To begin properly, we have to start at the end.

In getting to the heart of Barcelona's inability -- again -- to protect a three-goal lead, we have to start with Liverpool's fourth. Bear with me.

Long ago, Liverpool had a boot room at their training ground. It was the place, literally, where the squad's football boots were stored. But because it was informal, warm, more communal than the manager's office, the Reds' coaching staff used to meet there.

Legend has it that it was the kingdom of wisdom. Men like Bill Shankly, Joe Fagan, Ronnie Moran, Roy Evans and Bob Paisley used to share a cuppa, perhaps a crafty smoke, but above all they shared knowledge. They built a footballing empire on common sense and wit. They worked out how to keep their robust, daring, all-conquering team sharp, motivated and one step ahead of the competition.

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A phrase grew out of that boot-room culture, one that Paisley used to repeat to his footballers, who won him six league titles, five UEFA trophies among which were the jewels: three European Cups. "Find the dope," he told his men. "Find the dope!"

What that meant was that his Liverpool needed to be the team that never switched off. Like sharks, they hunted down the moment when a rival, even just one of them, switched off. Paisley told them that opponents, facing Liverpool's withering pace and passing, relentless, hungry and ambitious football, would tire -- mentally before physically. He told them that they would have a "dopey" moment -- be it just before half-time, when the referee stopped play or when the ball went out of play -- and that his Liverpool were required to pounce on that with ruthless, unerring predatorial lust.

Those words were originally uttered by Paisley decades before Trent Alexander-Arnold was born, but now you see where I'm going. Now you see why, in analysing Barcelona's remarkable collapse, their seemingly lemming-like procession over another cliff a year after Rome, it's important to start with the end.

The goal that put Liverpool in the Champions League final in Madrid, the goal that smashed Blaugrana dreams, was the goal that showed that Barca were the dopes.

Ernesto Valverde's side, like in the first leg, were getting the runaround. Liverpool's high press, wave after wave of red, was not only working, it was beginning to gnaw away at Barcelona's psyche. You could see the thought bubbles over Barca heads: "Oh, no! Not again!" Really, you could. Written in capital letters and with exclamation marks.

Valverde didn't pick the right XI. His contention that it would be "absurd" not to try to dominate possession was betrayed by his own unwillingness to risk Arthur.

The Brazilian has the one-touch/two-touch Xavi-esque skills to keep the ball moving, to turn the tide of pressure, to offer passing solutions. He did it at Camp Nou when Manchester United, briefly, threatened to do exactly this to Barcelona. He's small, inexperienced, not flawless -- but football brave. He represents the ethos this squad had, not too long ago, but is now patching up like a bicycle tire where the leaks have band-aids over them.

But, back to the dopes.

The Georginio Wijnaldum goals were coming; Liverpool were sending smoke signals, homing pigeons with written messages tied to their legs, spirit mediums were trying to get through to the Barcelona bench. But none of it was to any avail. They wouldn't listen. The Dutchman's up-and-at-'em style ruffled the Spanish champions so badly that he scored twice more quickly than it takes to brew a cup of tea.

And then the stage of the game arrived when either side could still qualify; the stage of the match when, even at 3-0 down, Barcelona still, if they were street smart, carried a huge advantage. One of their goals, thanks to playing away from home, would buy two of Liverpool's. If Valverde's mob had made it 3-1, then Liverpool needed two more -- more than they eventually scored even when they were flying.

But instead of this team boxing clever, instead of Barca understanding that however rattled and shaken they felt the advantage was not fully down the drain, they flunked the examination.

Just look at when Alexander-Arnold is about to relinquish the corner, he spots that Barcelona are the dopes and that Origi is wide awake. He takes such a brilliant centre that it's reminiscent of Lionel Messi. I hope they exchanged shirts at the end. It was witty, sharp -- streetwise. Paisley, in that blessed instant, would have been grinning down from the heavens.

But what of Barcelona? The reason I put so much emphasis on this moment isn't at all because it's the instant in which the lights went out, the dream faded and everyone was told to go home to misery and what Valverde called "penitence." No, the reason for the emphasis is that this is apparently the era of determination, of focus, of intensity at Camp Nou.

Nobody pretended that this is an era equal to the peak of Messi, Dani Alves, Xavi, Andres Iniesta, David Villa, Carles Puyol, Sergio Busquets and Eric Abidal. Nor did anyone, least of all Valverde, pretend that Barcelona had a coach for all the ages like Pep Guardiola is, was and will always be.

However, this is an epoch when Valverde has set up his team, all season, to work to cover Busquets' lack of running power. This is the Barca era when blue-collar concepts like a twin organising midfielder system -- doble pivote, in Spanish football language -- was not only an antidote to ageing and to lower grade possession play, it was specifically implemented to protect Barcelona against the maulings, like this one, that had been suffered in the past few seasons against Paris Saint-Germain, Juventus and Roma.

Above all -- and this casts in a terrible light on how Barcelona were thinking and behaving when that fourth goal went in -- Valverde's Barca were, they told us, united in a desire to follow flag-bearing Messi to glory, possibly to Treble glory. Lesser members of the squad were "getting gritty," working harder, inspired by the genius of the Argentine No. 10. All season long, Barcelona have been acting 180-degree opposite to the 79th minute when they went to sleep at Anfield.

All those themes of focus, determination, intensity and aggression have been true enough to arrange an eighth title in 10 years, to put Barcelona into what now looks like a slightly precarious Copa del Rey final. They also put Valverde's Blaugrana within one more goal of the Champions League final, whether in defeat at Anfield or the error of Ousmane Dembele, who must now be feeling a hot, red burn of humiliation at his feeble miss to extend the lead to 4-0 at Camp Nou last week.

The communal attitude, the "all for one" ethic, the grit, the "let's do it for Messi" level of physical commitment: those attributes that have made Barcelona dominant in Spain again this season, they'd never have allowed the Keystone Kops moment of which Alexander-Arnold and Origi took advantage. What happened? How did these hard-nosed, mission-inspired, elite professionals allow their pants to be pulled down?

When the corner was taken, Barcelona were statuesque. Nobody was alert, nobody was on the move; mild discussions about positional duties were taking place. It looked like any request for a teammate to move a few yards needed to filed in triplicate and signed off a day before. Liverpool were brutally brilliant, Barcelona were bogged down in penalty-box bureaucracy.

Through fatigue, through the haunting memories of a previous three-goal lead tossed away, they had become the dopes.

Arguably, there were notable differences here from those three humiliations in Paris, Turin and Rome. While Barcelona took a beating and deservedly exited, they competed well enough this time that Messi wasn't a shadow, that clear-cut scoring chances were created and scorned and that Alisson -- who's now a hex for the Blaugrana -- needed to produce four very good saves from Messi (twice), Philippe Coutinho and Jordi Alba.

Last week, before the first leg, I wrote in this column that: "One reason for writing so scathingly about Barcelona's weekend performance is the clash against Jurgen Klopp's red machine already felt like Superman meeting kryptonite for the first time. In case you're not familiar with the 81-year-old extraterrestrial, he went by the name Clark Kent and has made millions for DC Comics and Hollywood filmmakers. He left mortals standing, but kryptonite mysteriously weakened him -- just as Liverpool's pressing, athleticism, high-tempo passing, three-man front line and height at set pieces can potentially do to Barcelona."

Frankly, though I was accurate, it feels like I underplayed it now. Barcelona's 43-year inability to knock Liverpool out of Europe continues. Guardiola's words this week that Klopp's Liverpool are the best rivals he has faced now look nothing like Manchester City self-congratulation on being English champions-elect, but more like a loud warning -- unheeded -- for Valverde & Co.