The day's worst kept secret came to fruition about midway through Paul McCartney's 12-12-12 Hurricane Sandy benefit performance as his regular band exited the stage and the onetime Beatle found himself backed by the living members of Nirvana -- drummer Dave Grohl, guitarist Pat Smear and bassist Krist Novoselic -- for a brand new song called 'Cut Me Some Slack.'

The track was an all-out rocker with some of the adrenaline-fueled energy that McCartney provided more of earlier in his career and Grohl and his band the Foo Fighters have built their catalog on. McCartney appeared to be delivering a slide-like guitar sound while Novoselic and Smear rocked happily alongside him. Meanwhile, Grohl was his typical flurry of limbs and hair behind the drum kit, especially during the lengthy jam that came in the latter portion of the song. After the show, the Foo Fighters tweeted that the song is the first new track from Grohl's upcoming 'Sound City' documentary.

The Nirvana members weren't the only guests in McCartney's set as he welcomed Diana Krall to the stage to help out on piano for his 'Kisses on the Bottom' favorite, 'My Valentine,' a song that McCartney penned for his wife during a rough patch of bad weather that he felt seemed appropriate for the show.

McCartney fans got pieces from throughout his history as a musician. He opened the show by exclaiming, "I love New Yooooork," before launching into the 'White Album' classic 'Helter Skelter.' He would later revisit the 'White Album' by performing 'Blackbird,' a track McCartney said he penned to help people get through the struggles in life that still remains relevant today. The other Beatles track in his set was 'I've Got a Feeling' from 1970's 'Let It Be' album.

The vocalist also rolled out a pair of Wings songs, as the group performed the melodically pleasing 'Let Me Roll It' and 'Nineteen Hundred and Eighty Five,' both from the 'Band on the Run' disc. The singer would return to the piano to finish out his set with the explosive performance of his James Bond movie theme, 'Live and Let Die.'

Even though his performance was complete, McCartney wasn't ready to exit the stage as he welcomed, hugged and shook hands with a number of local police and emergency crews who were among the heroes in the aftermath of the Hurricane Sandy natural disaster. While McCartney was still greeting and thanking the city officials piling onto the stage, Alicia Keys was welcomed back out to complete the night with 'Empire State of Mind.'

The 12-12-12 Hurricane Sandy benefit show raised money for the Robin Hood relief fund. For more information or to make a donation, check here.

Watch Paul McCartney and Nirvana Unite for 'Cut Me Some Slack'

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