Although this was a mismatch on paper, Brighton were hardly overawed by their Premier League opponents and immediately took the match to the Gunners. It was the Seagulls who had the first great chance of the game, with Wojciech Szczesny forced into an excellent save from Ashley Barnes' 15th-minute effort.
But the visitors went ahead shortly thereafter, with Tomas Rosicky leading a vicious counterattack. The ball found Lukas Podolski and then Olivier Giroud, who didn't even look up as he took a touch and then smashed a shot into Caspar Ankergren's top corner.
If Arsenal had expected the opener to take the sting out of the match, they were sadly mistaken. The hosts had the ball in the back of the net within minutes via Leonardo Ullua following mistakes from Andre Santos and Szczesny only to see the tap-in ruled out for offside, but there was no such let-off for the Gunners when Barnes took advantage of a static defence to power home a corner in the 33rd minute.
The visitors went on the attack again in the second half, and Podolski rattled Ankergren's crossbar with a superb free kick before a long pass from Abou Diaby saw them reestablish their lead. Giroud still had plenty to do as the ball dropped over his shoulder, but a divine first touch set the former Montpellier striker up for an acrobatic finish to make it 2-1.
Again, Brighton refused to give up. Gus Puyot's side had been exploiting Andre Santos' presence on the Arsenal left all match, and the hosts took advantage of the Brazilian's woefully incompetent display for the equaliser, which saw the so-called left back refuse to defend a cross from Barnes that allowed Ullua to mark his debut in style with a flying header.
Hoping to avoid a replay, Arsene Wenger committed both Theo Walcott and Jack Wilshere to the fray, and the move paid dividends in the 85th minute. An impotent punch from Ankergens presented the former with a volley at the edge of the box presented the forward with a straightforward volley, and although Adam El-Abd tried a desperate block the ball merely cannoned off his shoulder and into the back of the net.
The hosts couldn't recover from this third setback. Substitute Craig Mikkail-Smith made a late plea for a penalty only to be denied by Michael Oliver as the impressive Seagulls crashed out of the cup.