Corden labelled it "the most attractive story" he has ever heard, before the pair perform a version of it. So I woke up and was, 'What was that?
Later, McCartney and his band surprised a small group of locals at Liverpool's Philharmonic Pub with a 13-song set that included "A Hard Day's Night", "Back in the U.S.S.R" and his new single, "Come On To Me".
After getting back in the auto, Corden says Sir Paul's music is "so full of positivity and joy", and "more relevant now today than it's maybe ever been".
"That's the most attractive story I've ever heard", Corden said before the two broke out in an emotional duet of "Let it Be" that brought tears to talk show host's eyes as he drove.
The Beatles legend gamely rode shotgun for an epic edition of Carpool Karaoke - running more than 20 minutes - that aired on the final night of The Late Late Show's week of London-based shows. What'd she say? Let it be? "And it keeps being relevant".
Then they pulled up at the home where he lived during his late teen years, and where he and John Lennon wrote their hit songs. "I had to text him back and was like, 'No dude, that's not what we meant.' What we were trying to say was it was uncomfortable to sit in a vehicle and try to sing our own songs".
"If my granddad were here right now, he'd get an absolute kick out of this", Corden added, with tears still in his eyes.
Corden was lucky enough to be invited onstage to join McCartney to bring down the house - which contained a few stunned, teary-eyed fans - with an all-voices-on-deck rendition of "Hey Jude".
Later, he performs some Beatles hits after appearing from behind a curtain in a pub. "We're going to give the locals the surprise of their life", teased Corden. Posing as a regular bartender, Corden served a drink or two before asking a patron to select a song on the jukebox.
The unexpected concert continued as the music legend rocked out with a slew of beloved hits, while numerous patrons were moved to tears by the surprise.