MADRID — Karim Benzema could not have known, not consciously, what he was doing. It all happened too quickly, too chaotically, to be anything other than instinctual. He was standing on the edge of the Paris St.-Germain box. The ball slipped through a thicket of players. It was at his feet. He jabbed out a foot, a flash of movement, a tic, a twitch. And then everything melted around him.

Benzema raced off to the corner of the Santiago Bernabéu, its remodeling still a work in progress, where the new is slowly emerging from the old. His Real Madrid teammates sprinted from all directions to join him, to swarm him, to swallow him. David Alaba grabbed a plastic folding chair and brandished it above his head. The stands above writhed and shook, the crowd rendered delirious by witnessing the impossible.

Not quite 20 minutes earlier, Real Madrid had been out of the Champions League. As good as gone, anyway. The team that prides itself as the Kings of Europe — as a banner unfurled by the club’s ultras before the game put it — looked old and tired, caught in the megawatt glare of P.S.G.’s star power.

It was not just that Kylian Mbappé had scored, extending the French side’s lead to two goals on aggregate; it was that he had seen two more disallowed for offside, one of them the sort of moment only the true greats can conjure, somehow leaving Thibaut Courtois, Real Madrid’s goalkeeper, sprawling on the grass despite not even touching the ball.

Mbappé’s every move flickered with menace, fizzed with energy. Éder Militao, the defender tasked with shadowing him, is no slouch, but he had spent much of the evening heaving for air, staring at the Frenchman’s heels. Neymar, too, was starting to drift and to dance, picking holes and pulling strings. For an hour, one team looked like the future, and the other like the past.

The Bernabéu sensed it, too. Half the stadium remains scarred by engineering work, but the club had found a way to cram in 61,000 fans, its largest crowd in two years. They had gathered hours beforehand, lighting flares and throwing firecrackers on the streets running from the Paseo de la Castellana, bravado erasing the doubts and the fears.

They had found it within themselves to applaud Mbappé when his name was announced — they might be seeing more of him, after all — but this was not what they had come to see. Real Madrid is not supposed to be the foil for someone else’s exhibition. The grumbles and the groans, muted at first, grew louder with every P.S.G. pass.

And then, from nowhere, everything changed. Gianluigi Donnarumma dawdled on the ball; Benzema shoved him aside. The ball fell to Vinicius Junior, who returned it to Benzema, a few yards from goal. Suddenly, Real Madrid had a glimmer. In this competition, a glimmer is all anyone needs.

The knockout stages of the Champions League have, in recent years, made a habit of producing the unthinkable; it happens so frequently now that the only conclusion is that the spectacular is hard-wired into the competition’s underlying code. Through some combination of factors — the high stakes, pressure and critical mass of talent — it has become the most fertile breeding ground imaginable for the spectacular.

Nobody is immune. It has happened to Ajax, Manchester City, Bayern Munich and Real Madrid itself over the years. But whether it is through correlation or causation, it does seem to happen to both Paris St.-Germain, and to Lionel Messi, rather more than might be expected.

For P.S.G., that first goal from Benzema carried with it an echo of the failures that have marred its desperate, expensive attempts to win this competition: the ransacking of the Parc des Princes by Manchester United and, most of all, the 6-1 defeat to Barcelona in 2017, the game the club has spent hundreds of millions of dollars trying to forget.

Messi, too, seemed as if he had seen a ghost. He was present, after all, for Barcelona’s collapses in Rome in 2018 and at Anfield in 2019; he was on the field the day the greatest club team in history succumbed, 8-2, to Bayern Munich in 2020. He had been powerless, then, and he seemed powerless now.

He had, in truth, been a peripheral figure for much of the game, flickering to life only occasionally, overshadowed even when P.S.G. ran rampant by the vibrancy and the youthfulness of Mbappé. As soon as Real Madrid scored and the Bernabéu roared, though, he seemed to sink from view completely, a callow and diminished figure, the greatest force of agency soccer has ever seen apparently resigned to his fate.

When it came, it hit him, and his teammates, like a wave, shifting the ground from beneath their feet in the space of no more than 120 seconds. Luka Modric, a veteran raging more effectively against the dying of the light, fed Benzema, who smuggled the ball past Donnarumma, drawing Madrid level on aggregate.

The noise from the celebrations was still rattling around the Bernabéu when the ball broke for Benzema and he jabbed out a foot and he raced away, arms outstretched, into a squirming mass of white.

There was, even then, still time for P.S.G., for the most expensive squad in the history of soccer to find a goal against a team that it had pinned against the ropes only a few minutes earlier, but it almost seemed too distressed, too dazed, to believe it.

Mbappé, Neymar and Messi, that strike force of the best there was, the best there is and the best there might yet be, prowled the field forlorn. They knew how this ended: with lingering shots of them, heads bowed, eyes haunted, staring at the ground or gazing into the middle distance. By the time the final whistle blew, as Real Madrid’s players collapsed onto their backs and P.S.G.’s crumpled to their knees, Messi was nowhere to be seen. He had slipped from the field without a glance, without a word. It was possible, in the bedlam, to forget he had ever been there at all.