Sir Paul has been really busy. First he performed a surprise show at Irving Plaza, then he performed (some would say, badly) at the "Saturday Night Live" 40th anniversary special. And then at an after-party last night, he played again. This time with Taylor Swift and Jimmy Fallon.

TMZ has video!

Swift is on guitar and singing her hit "Shake It Off" and McCartney is playing guitar, too. And then the video cuts to Swift and Fallon singing the Beatles hit "I Saw Her Standing There." Again, McCartney is on guitar.

We want to know what's going through his head. Here's what's going through Taylor's and Jimmy's:

Fallon: Another night, another duet.

Swift: OM-Effing-G my life is perfect!