Nani's 94th-minute winner completed a superb comeback by Manchester United as they overcame derby rivals Manchester City to clinch the Community Shield.
With no extra-time to be played and penalties looming, the Portuguese winger pounced on some hesitant City defending to run on to a Wayne Rooney clearance, round Joe Hart and slot home.
City had led 2-0 at half-time after a shaky start by United's Spanish keeper David De Gea saw him beaten by Joleon Lescott's header and Edin Dzeko's long-distance drive.
But United, beaten at Wembley on their last two visits - by Barcelona in the Champions League final and City in the FA Cup semi-final - responded by taking the game to their neighbours and ultimately they got their reward.
Chris Smalling reduced the deficit when he turned in Ashley Young's free-kick before Nani levelled before the hour mark by finishing off a stunning passing move.
City held firm and even threatened to retake the lead when Adam Johnson tested Aguero, but it was United who laid down the marker for the forthcoming Premier League campaign, beginning the defence of their title by collecting more silverware.
In truth, Sir Alex Ferguson's men could have been out of sight by the time City scored, with new signing Young quickly looking at home down the left flank, but they could not make the most of their vastly superior possession.
Young had a shot deflected narrowly wide before Lescott blocked Smalling's volley on the line, but United's best efforts were free-kicks from Rooney and Nani which flew narrowly off target, the latter deflecting off Dzeko.
City, in contrast, were lethargic in every area of the pitch and their main contribution in the first half-hour was to keep referee Phil Dowd busy, with Micah Richards among four of their players booked early on, in his case for a wild lunge on Young.
At that stage, debutant De Gea had been given next to nothing to do in United's goal - which was why City's opener was a huge surprise when it arrived in the 38th minute.
David Silva whipped in a cross from the right and Rio Ferdinand appeared to leave the ball for his keeper, who had stopped, allowing Lescott to glance his header home.
If there were doubts over who was culpable on that occasion, there were none when Dzeko made it 2-0 on the stroke of half-time.
United's defence backed off to allow the Bosnian space to shoot from 25 yards out and, although there was plenty of pace behind his shot, De Gea appeared to react too slowly and dived after it had flown past him.
Ferguson's response to seeing his side trail at the break was to make three changes, bringing on another new signing Phil Jones and Jonny Evans to replace Ferdinand and Nemanja Vidic at the back, and introducing Tom Cleverley into his midfield.
His changes brought almost instant dividends, with United pouring forward following the restart.
Smalling reduced the deficit from close range when Young's free-kick evaded Lescott and, six minutes later, Nani levelled when he rounded off a quick-fire passing move involving Rooney and Cleverley.
It seemed United's momentum would overwhelm their neighbours but City, to their credit, stood firm. United remained the more dangerous with their rapid attacks but they did not force Joe Hart into another save before striking right at the death.
Rooney hooked clear from a City corner and with Vincent Kompany standing off, Nani nipped past Yaya Toure and kept his cool to beat Hart and settle an enthralling contest.