In the mid-’70s, long after The Beatles had officially parted ways, John Lennon recorded a series of demos straight to cassette tape in his home in New York City. After his death in 1980, his wife, Yoko Ono, passed the tapes to the remaining Beatles, and in 1995, Paul McCartney, Ringo Starr, and George Harrison began work on a trio of new Beatles songs born from Lennon’s sketches. You’ve heard a couple of them before—“Real Love” and “Free as a Bird”—but one of them never came to fruition during those sessions. “When we started ‘Now and Then,’ it was very difficult because John was sort of hidden in a way,” said Starr in a documentary about how The Beatles’ final song came together. “In John’s demo tape, the piano was a little hard to hear,” said McCartney. They put together some of the song’s building blocks, including having Harrison record guitar parts for the song. But they couldn’t properly isolate Lennon’s vocals from his piano. So “‘Now and Then’ just kind of languished in a cupboard,” McCartney said. Fast-forward to 2022. Using machine-learning technology that director Peter Jackson’s team had developed for the 2021 Beatles documentary Get Back, engineers were able to separate Lennon’s voice from his original demo of “Now and Then,” finally allowing the remaining Beatles to finish the track. “It's like John’s there,” said Ringo of the now-crystalline vocal track. “It’s far out.” Paul and producer Giles Martin (son of Beatles producer George Martin) were then able to assemble John’s 1970s vocals with George’s 1995 guitar lines and Paul and Ringo's new vocal and instrumental parts, as well as a string arrangement, to complete the song. Bearing the signatures of all four Beatles, it’s a doleful ballad and a poignant good-bye that resonates far beyond its initial intention. Hear The Beatles’ last song alongside their very first—1962’s “Love Me Do”—and watch the trailer for the documentary about how “Now and Then” was made.