Amazing. Lightning rarely strikes once against Barcelona. Here it struck twice. Unbelievable.
One-nil down to the best team on the planet, struggling to see the ball, let alone the goal, Arsenal responded in sensational style, scoring twice in five minutes to stun Barcelona.
This was a turnaround, a victory born of resilience.
Driven on by the outstanding Cesc Fabregas, given energy by young Jack Wilshere, Arsenal refused to surrender after David Villa gave Pep Guardiola's team of all the talents a deserved half-time lead.
Barcelona still enjoyed possession for long periods of the second half, but Arsenal used the ball brilliantly when it was on loan to them. First Robin van Persie struck, exploiting bad positioning by Victor Valdes, but the breakaway second was a gem.
Samir Nasri set up Andrey Arshavin, whose goal gives Arsenal hope for the Nou Camp second leg, particularly with Gerard Pique suspended. Yet for so long, Arsenal had been outplayed and outpassed by the cultured visitors.
Barcelona are the princes of pressing, masters of forcing opponents into ceding the ball and then working their clever passing moves, but Arsenal worked hard to contain them, holding out for 26 minutes.
The opening stages shimmered with hope for the team slightly harshly dubbed "Barca Lite". Briefly. But it needs recording. Arsenal stood toe-to-toe with the heavyweights of the Beautiful Game and traded combinations. In truth, Arsenal started both halves well.
In front of Fabio Capello, Theo Walcott's pace and determination delighted the Arsenal fans as the game unfolded. Wilshere looked confident in possession. Yet such is the composure and movement of this magnificent team of Guardiola's that danger levels remained high, even before their goal just before the half-hour.
Any laxness in possession was seized upon. Barely had the opening whistle faded in the cold north London air, Barcelona raced upfield, Messi driving through the middle, his elegant progress stopped cynically by Alex Song.
Some of Barcelona's movement in the centre was utterly magical, beautifully simple too in the way the likes of Xavi turned away from Nasri and then Sergio Busquets dummied to elude Wilshere. Even Guardiola showed his first touch, controlling a loose ball.
For a while, Arsenal defended well, all in red helping out, although the returning Nasri often let Dani Alves go. Others impressed more with their contributions in keeping Barcelona at bay. Song tracked back on Messi.
Laurent Koscielny nicked the ball off Xavi. But that link-up between David Villa and Messi was a constant menace. After 15 minutes, Messi darted on to Villa's pass, ghosted through, committed Wojciech Szczesny, but chipped the ball just wide.
This was a treat for purists. Class was seen all over the Emirates from the expert way Pique took the ball off Fabregas, from the deft way Xavi kept sweeping the ball right to Alves and also in the admirable technique and belief of Wilshere, who wrongfooted Pedro with a move that deserved an 'ole'.
The pressure around Arsenal's goal built inexorably, demanding constant fire-fighting work from Arsenal players. Walcott charged back to win the ball from Maxwell. Wilshere, tireless between the boxes, blocked Villa's cross.
Then, as the Emirates dared to dream, Wilshere invited Walcott to run through the middle. Fabregas broke to his right and Walcott swiftly found him. The angle was too tight to shoot, so Fabregas lifted the ball over, trying to pick out Van Persie, but Eric Abidal headed clear.
Barcelona's response was typical. Messi teased the ball through the Gunners' broken ranks and there was Villa, sprinting through, clearly onside, to place the ball between Szczesny's legs and in.
Arsenal, commendably, refused to fold. Even when Pedro almost scored, his backheel saved by Szczesny, Arsenal kept believing. Even when Barcelona played keep-ball so effortlessly, Arsenal kept going.
Van Persie wasted a wonderful chance just before the interval, shooting wide, but Arsenal regrouped in the dressing-room, remerging with renewed vigour.
Fabregas stepped up, driving Arsenal forward. Nasri sprang to life.
Pique was placed under pressure, responding with a foul that rules him out of the Nou Camp return. Fabregas then released Nasri down the inside-left channel, and the Frenchman swept in a ball to the near-post. Pique, just sliding in ahead of Van Persie, cleared.
This was much, much better from Arsenal. Fabregas was again to the fore just after the hour-mark, rolling the ball inside to Van Persie, who brought a fairly routine save from Valdes.
With Arsenal enjoying more possession, Guardiola twisted, withdrawing Villa and sending on the more defensive Seydou Keita. Messi went up top but the movement remained kaleidoscopic.
Wenger also acted, sending on Arshavin for the booked Song, a move that was to bring such joy. Amazingly, occasional signs of frustration crept into Barcelona's play, Messi shaping to go in foot up on Koscielny.
Arsenal needed a Plan B. Walcott was off the pace, soon replaced by Nicklas Bendtner. Wilshere continued to impress, dribbling past Xavi and Pique before Abidal flew in to quell the danger.
Arsenal fans kept singing, kept showing their support and their belief was rewarded. When Gael Clichy lifted the ball over Pique, Van Persie found himself in space but the best option seemed a cross.
Valdes made his mind up. When the Barcelona keeper drifted off his near-post, Van Persie pounced, driving the ball in. Great finish. Poor keeping.
There was more, unbelievably so. When Nasri raced down the right, Barcelona defenders failed to spot Arshavin racing into the box. When Nasri's cut-back came, the Russian pounced, stroking the ball home from 15 yards, sending Arsenal fans into rapture. (© Daily Telegraph, London)