A demo of the song, featuring Denny Laine on lead vocal, was included as a bonus track on the Archive Collection reissue of Wings at the Speed of Sound.

[5][6] The lyric namechecks several famous people, between friends and relatives of McCartney who, without a justified reason, knock on the door or ring the bell of his house and he exclaims "Let 'Em In".

Phil and Don of the Everly Brothers are named (the duo covered "Keep A-Knockin'" on their self-titled album), along with Martin Luther,[7] who is said to have hung his "Ninety-five Theses" on a church door.

An Uncle Ernie is also named, being the character Ringo Starr sang in the London Symphony Orchestra's recording of the Who's rock opera, Tommy.

The song makes use of the piano, drums, brass, including a trombone solo, and wind instruments, featuring flutes, as well as backup vocals from Linda and other members of Wings.