Former Saints star Theo Walcott bagged the second club hat-trick of his career as Arsenal completed an extraordinary comeback to beat Reading 7-5 in a barely believable Capital One Cup fourth-round tie at the Madjeski Stadium.
A youthful Gunners line-up recovered from 4-0 down inside the opening half to snatch an unlikely win when Marouane Chamakh completed the scoring in the 31st minute of extra-time.
Remarkably there was still more to come as Marouane Chamakh put the seal on a breathless encounter moments later when he lobbed beleaguered Reading goalkeeper Adam Federici for his second.
Arsenal had only forced extra-time with the last kick in normal time when Carl Jenkinson fired home after Chris Gunter had blocked Walcott's effort.
It was a pulsating end to a contest that had looked over following the home side's early blitz.
Veteran Jason Roberts got the ball rolling before a Laurent Koscielny own goal and Mikele Liegertwood's strike, which debutant goalkeeper Damian Martinez helped into the net, fired Reading 3-0 up after just 20 minutes.
The Royals were in dreamland when Noel Hunt then outjumped his marker on 37 minutes but a Walcott goal on the stroke of half-time gave Arsenal the faintest of hope.
Olivier Giroud boosted belief when he headed in moments after coming on before Koscielny made up for his earlier blunder to leave Arsenal with only the six minutes of injury time to level.
They did, in the final minute of added time, although there was some confusion over the goal.
Theo Walcott's initial strike was blocked by Gunter, Jenkinson ensuring extra-time by smashing in the loose ball.
Arsenal led for the first time when Chamakh scored his first goal in almost a year after 13 minutes of extra-time but Pavel Pogrebnyak continued the drama with a 116th-minute leveller.
But Reading saw the game snatched away again by Walcott and Chamakh in injury-time.