Hamilton held a 7.3secs advantage over the field as lap 35 ticked by, with the top four comfortably strung out and only the Webber/Alonso battle creating tension, but Mercedes' earlier call for primes began to make more sense as both Red Bulls and Button opted for the same choice at their next stop shortly afterwards. Alonso and Hamilton, meanwhile, took on more super-softs and, even if the order did not change a great deal, it set up a fascinating scenario for the end of the race with those on the harder rubber potentially running to the flag without requiring another change.
Just as Hamilton appeared poised, pit-stop or not, for a second win as many weekends, however, the #3 McLaren was spotted spinning at the turn six chicane. The Briton, again beginning to see graining on his left front after just five laps, was also caught out by a quick shower that affected the rear of the circuit. Putting that susceptible tyre onto the inside kerb of the left-hand element was enough to pitch the car around and, even though the Briton was able to flick himself back into the race, the delay was enough to allow Button through, even though hitting the brakes to avoid passing another car under yellows saw the two Britons immediately running together.
Vettel, too, closed onto the tail of the two McLarens, but was unable to find a way through, allowing the Britons, who opted to stay out despite the rain, to battle through the next couple of laps. While Alonso stopped for primes, and Vettel skated off again at turns two and 13, Hamilton and Button went at it hammer and tongs. The latter's lead lasted until turn two, where he ran wide and allowed Hamilton through, but the #4 was back in front next time around as Hamilton stopped for a return to intermediates, along with Alonso, Webber and Rosberg.
The move proved to be the wrong one and, as Button survived on his hard slicks, those that stopped quickly found that there was not enough precipitation to warrant the treaded rubber, necessitating another stop if they were to make it to the end. To make matters worse for Hamilton, he was also called for a drive-thru' penalty, after the stewards deemed his spin-turn detrimental to Paul di Resta's race after the Scot was forced off the road, effectively ending his pursuit, not only of victory, but also of a podium finish.
Webber admitted that running for two laps on the intermediates had made Red Bull look a little stupid, but insisted that another five minutes of rain would have transformed the decision into one of genius. With Alonso quickly assuming the third position vacated by Hamilton, the Australian passed the other Ferrari to claim fourth, but then found himself coming under pressure from the McLaren in the closing stages, and had no answer to a determined Hamilton as the Briton took advantage of thick traffic to move back onto the fringe of a top three spot.
Behind them, Massa was comfortable in sixth as Rosberg's ill-fated tyre choice briefly relegated the German out of the points, while di Resta was able to hold off the Toro Rosso of Sebastien Buemi, who had the recovering Mercedes on his tail at the flag. The second STR of Jaime Alguersuari rounded out the scorers, despite clashing with Kamui Kobayashi's Sauber as he tried to follow his team-mate through into turn one with nine laps remaining.
Out front, however, there was little doubt as to the result, as Button once again proved to be the master of tricky and changing conditions. His three previous wins for McLaren - in Australia and China last season and Canada last month - have all come in similar circumstances, and the 2011 Hungaroring success mirrored that of his first ever GP triumph, at the same circuit, five years ago. Vettel remained a threat for a couple of laps, but the McLaren had been the better car all afternoon, and Button, who had been forced to retire in each of past two races, was able to eke out a bigger gap before slowing down to celebrate his achievement with the fans on the final lap.