It was a bright opening for the Royals, who won a couple of early corners thanks to some sustained possession - the second of which was headed in at goal by Sean Morrison, only for the ball to cannon back from the bar onto 'keeper Damian Martinez and away.
Jay Tabb was next to cause a flutter amongst the Gunners backline, slaloming into the area after ten minutes and showing great balance to stay on his feet - before seeing a low effort well blocked for a corner.
The warning signs were their for Arsenal, but perhaps not headed - as Reading took the lead in the 13th minute. A well worked move ended in a magnificent cross from the left boot of Hal Robson-Kanu, and an equally adept finish from Jason Roberts - who smashed into the roof of the net from six yards.
That wasn't enough to appease the rampant Royals - who continued to bombard the visitors with wave after wave of attacks, before eventually doubling their lead twenty minutes in.
Some neat work by Garath McCleary allowed Chris Gunter to gallop into the box, before slamming in a cross that cannoned off Laurent Koscielny and into the net.
And still the pressure came.....as did the third goal of the evening, courtesy of Mikele Leigertwood. An exquisite drop of the shoulder from the boyhood Arsenal fan was enough to give Reading's number eight enough room to fire in a left footed shot, and a slight deflection flummoxed Martinez and rippled the net once again.
The Gunners struggled to mount a response, as the game then petered into its first lifeless patch in the middle of the opening half - before Noel Hunt sprung things into life again with a fizzing daisy cutter from 25-yards, which was well dealt with by Martinez.
If that was close, the Irishman need not have worried - as his trademark spring aided him greatly in notching the fourth of the evening. A brilliant threaded pass by Leigertwood allowed McCleary the space to whip in a delectable cross - where the Royals number ten headed brilliantly inside the far post, despite another valiant effort from Martinez.
, in truth, didn't come close to being tested until the final three minutes of the half - as Marouane Chamakh's tame effort was easily gathered by the Australian international.
Emmanuel Frimpong's long range strike had a little more venom to it, but thankfully for the home support - flew wide of the post.
The increase in pressure would eventually lead to a sting in the tail for McDermott's men, as Theo Walcott showed trademark pace and poise to dash in on goal and loft the ball calmly past Federici to give his side a glimmer of hope.
The second half started in an equally explosive nature, as Robson-Kanu's header was well stopped by Martinez, before a stray Arsenal boot almost created their second goal of the game - as the ball flew narrowly over the bar for a corner kick.
In the fiftieth minute, some good wing play by Roberts allowed the experienced front man to cut into the left hand side of the area, before squaring for Leigertwood - who blazed over when positioned well 12 yards from goal.
The rapid legs of Theo Walcott posed a constant threat on the breakaway, as the England international responded by haring down the left wing before forcing a good save from Federici at his the post.
The highly rated Serge Gnabry also looked dangerous when given time on the ball, as he smashed a low effort just past the far post moments later.
Whole hearted claims for handball and a penalty against Chris Gunter were waved away by referee Kevin Friend shortly before the hour mark, as Chamakh managed to find himself in the book for the resulting protests - whilst Walcott came within inches of bagging a brace seconds later as he diverted a Jenkinson cross just off target.
Arsene Wenger elected to make a double substitution on the hour mark, with Olivier Giroud and Thomas Eisfeld introduced - and it quickly paid of as Giroud clawed another goal back with a thumping header from Walcott's corner kick.
The French international almost grabbed a second ten minutes later, as his well struck left footed drive was well palmed away by Federici.
Brian McDermott responded with a double change of his own, with McCleary and Hunt making way for Jobi McAnuff and Pavel Pogrebnyak.
Giroud was a huge aerial threat for the Royals backline, and another powerful header from the visitors' number 12 forced Federici into a magnificent save 13 minutes from time - as the Aussie dived low to his right to palm away.
He was involved again seconds later, as the red waves of attack continued - and the targetman's outstretched boot came a whisker away from converting a cross-some-shot at the far post.
Some wiley work from Jason Roberts would briefly give the home side some respite, but with the clock just a minute away from stoppage time hearts were well and truely in mouths - as Laurent Koscielny converted with another thumping header.
The nail biting would continue into four minutes additional play - as the irrepressible Walcott ducked and dive on the left before seeing a shot deflected just off target.
But, with the referee surely seconds away from blowing for full time - those hearts were broken.
Walcott it was again, who somehow clipped the ball over the line despite valiant attempts from Shorey to hook away and prevent another 30 minutes of football.
Reading had to somehow pick themselves up and go again.
Jobi McAnuff was determined to rally the troops early on in extra time - as he drove at the heart of the Arsenal defence before forcing an excellent stop from Martinez.
At the other end Walcott was at times unstoppable, again dancing through a crowd of players before laying off for Arshavin - who could only slice wide from the edge of the box.
Morrison was attempting to equal the height of Giroud in the Arsenal box, as he headed a corner over the bar midway through the half.
That being the case, after such a heart breaking comeback - a fifth Gunners goal seemed almost inevitable, and it came from the boot of Chamakh, who clipped a well struck shot past the out-stretched arms of Federici from 20 yards.
HALF TIME - EXTRA TIME
Federici had to be alert again early in the second period of extra time to deny Walcott his hat-trick with another solid stop - whilst at the other end Morrison again headed over from Shorey's corner.
The giant centre half came on step closer from Shorey's delivery, forcing Arsenal's stopper to save well at the near post - though the linesman's flag was raised.
However, and under Brian McDermott - there always seems to be a however, the Royals were yet again not to be beaten as they somehow clawed their way back into a remarkable evening.
McAnuff's shot was scuffed, but Pogrebnyak - reacted first - slamming past a hapless Martinez to make it 5-5.
Naturally that wasn't the end of the drama, and ultimately - it was to be Arsene Wenger's night.
Walcott grabbed his hat-trick in the process - smashing home from close range after Arshavin's breakaway.
Chamakh's late lobbed goal after an error of judgement from Gunter was almost an irrelevant footnote, as the Gunners booked their place in the quarter final
It was a night that will probably never be forgotten for those that watched it, and unfortunately for Royals fans - not for the right reasons.