As soon as Marcus Thuram found the net after 30 seconds - the second earliest goal Inter have scored from the start of a UCL game after Dejan Stankovic's strike against Schalke in April 2011 (26 seconds) - the blue touch paper was lit.
Only Alexandre Pato's strike for AC Milan in September 2011 (24 seconds) has seen Barca concede quicker in the competition, and the shock of going a goal down so early silenced what had been a raucous Estadio Lluis Companys crowd.
When Denzel Dumfries fired home a spectacular second on 21 minutes you could've heard a pin drop inside the stadium, aside from the Italian contingent who undoubtedly couldn't believe their luck.
After all, Inter had ceded the advantage in the Serie A title race over the past couple of weeks, had been dumped out of the Coppa Italia by rivals Milan, and had won just two of their last eight in all competitions before facing the Catalans.
Stunning performance from Barcelona centurion Lamine Yamal
Dumfries would go on to add another in the second half (Two goals, one assist), becoming the first Dutch player to both score and assist in a Champions League semi-final since Wesley Sneijder in April 2010, coincidentally against Barcelona.
The game should've been over as a contest after the second goal, but Barca's centurion, Lamine Yamal, had other ideas.
His goal on 24 minutes to get the hosts back into the game was a thing of beauty, and it meant that the 17 year and 291 day old teen became the youngest ever scorer in a Champions League semi-final, beating a record previously held by Kylian Mbappe (18y, 140d).
Despite Lamine's young age, his importance is evidenced by the fact that only Jules Kounde (107) and Robert Lewandowski (104) have played more games for the club since the Spaniard's debut on 29 April 2023 as a 15-year-old.
Raphinha continuing superb Champions League form
Raphinha, another player who has really stood out for Hansi Flick's side this season, offered up another assist, this time for Ferran Torres, before half-time to ensure the teams went in all square at the break.
It was the Brazilian's eighth in the 2024/25 edition of the tournament, just one behind Luis Figo's nine in 1999/00, which is currently the most in one campaign.
Only Cristiano Ronaldo now has more total goal involvements in a single Champions League campaign (21 - 17 goals, four assists in 2013/14) than Raphinha this season (20 - 12 goals, eight assists).
Despite Inter naming the seventh oldest starting XI for a UCL semi-final game (30y & 56d), they were good value for their draw, and may well feel that they now have the advantage in terms of progression into the final against either Arsenal or Paris Saint-Germain.
All to play for in Milan
Unlike in their quarter-final against Roma in 2018 and their semi-final against Liverpool in 2019, Barcelona don't hold a healthy lead so complacency is unlikely to creep into their play as it did then. Having to work harder to make the final could therefore be of benefit.
They will need to improve on how they break down a dogged Inter defence however - one who made 41 clearances in Wednesday night's first-leg - whilst keeping things tight in their own backline.
When you consider that Wojciech Szczesny didn't actually make a single save in the game, it beggars belief that the visitors were able to plunder the Barca goal with relative ease.
Indeed, it was only with regard to tackles made (14 to 11), duels won (56.3% to 43.7%) and dribbles completed (71.4% to 55%) where Inter found themselves on top.
In every other aspect - including possession (71.5% to 28.5%), accurate passes (622 to 204), shots (19 to 7) and final third entries (85 to 24) - Barcelona were emphatically better.
A tale of two strikers with Ferran thriving & Martinez a doubt
The records will show, however, a 3-3 draw as the final result, the joint highest score draw in a Champions League semi-final along with the 3-3 between Dynamo Kyiv and Bayern Munich back in 1998/99.
Inter will know that if they stop Lamine Yamal, they still have a great chance to progress into the final, but they can't let the youngster work his magic in their own box. His 17 touches inside Inter's penalty area were 11 more than any other player on the pitch and hints at the threat that Barca possess down their right-hand side.
The Italians would also do well to keep tabs on an increasingly confident Ferran Torres, because 'The Shark' has really come into his own in the absence of Robert Lewandowski.
More playing time has seen to it that the Spaniard is now finding the rhythm he's needed to produce his best work. Four shots on goal against Inter was the joint most (with Lamine), and whilst he only managed one goal in the first leg, he's unlikely to look a gift horse in the mouth in the return.
It isn't clear if Lautaro Martinez will be available for that game after he was substituted, but he offered nothing at all in the first leg other than being caught offside once, so may not be the biggest of misses after all.
Simone Inzaghi will now have a much clearer idea too of how to beat the tournament favourites, and in Denzel Dumfries and Marcus Thuram, they have their match winners - and the two busiest Inter players from the first leg.
With thousands of Nerazzurri to roar them home at San Siro, another brave performance will be expected and needed.
