Titled “Now and Then,” the almost impossible-to-believe track is four minutes and eight seconds of the first and only original Beatles recording of the 21st century. There’s a countdown, then acoustic guitar strumming and piano bleed into the unmistakable vocal tone of John Lennon in the song’s introduction: “I know it’s true / It’s all because of you / And if I make it through / It’s all because of you.”

More than four decades since Lennon’s murder and two since George Harrison’s death, the very last Beatles song has been released as a double A-side single with “Love Me Do,” the band’s 1962 debut single.

“Now and Then” comes from the same batch of unreleased demos written by Lennon in the 1970s, which were given to his former bandmates by Yoko Ono. They used the tape to construct the songs “Free As a Bird” and “Real Love,” released in the mid-1990s. But there were technical limitations to finishing “Now and Then.”

On Wednesday, a short film titled “The Beatles — Now And Then — The Last Beatles Song” was released, detailing the creation of the track. On the original tape, Lennon’s voice was hidden; the piano was “hard to hear,” as Paul McCartney describes it. “And in those days, of course, we didn’t have the technology to do the separation.”

That changed in 2022, when the band — now a duo — was able to utilize the same technical restoration methods that separated the Beatles’ voices from background sounds during the making of director Peter Jackson’s 2021 documentary series, “The Beatles: Get Back.” And so, they were able to isolate Lennon’s voice from the original cassette and complete “Now and Then” using machine learning.

When the song was first announced in June, McCartney described artificial intelligence technology as “kind of scary but exciting,” adding: “We will just have to see where that leads.”

“To still be working on Beatles’ music in 2023 — wow,” he said in “The Beatles — Now And Then — The Last Beatles Song.” “We’re actually messing around with state-of-the-art technology, which is something the Beatles would’ve been very interested in.”

“The rumors were that we just made it up,” Ringo Starr told The Associated Press of Lennon’s contributions to the forthcoming track in September. “Like we would do that anyway.”

“This is the last track, ever, that you’ll get the four Beatles on the track. John, Paul, George, and Ringo,” he continued.

McCartney and Starr built the track from Lennon’s demo, adding guitar parts George Harrison wrote in the 1995 sessions and a slide guitar solo in his signature style. McCartney and Starr tracked their bass and drum contributions. A string arrangement was written with the help of Giles Martin, son of the late Beatles producer George Martin — a clever recall to the classic ambitiousness of “Strawberry Fields,” or “Yesterday,” or “I Am the Walrus.” Those musicians couldn’t be told they were contributing to the last ever Beatles track, so McCartney played it off like a solo endeavor.

On Friday, an official music video for “Now and Then,” directed by Jackson, will premiere on the Beatles’ YouTube channel. It was created using footage McCartney and Starr took of themselves performing, 14 hours of “long forgotten film shot during the 1995 recording sessions, including several hours of Paul, George and Ringo working on ‘Now and Then,’” Jackson said in a statement.

It also uses previously unseen home movie footage provided by Lennon’s son Sean and Olivia Harrison, George’s wife, and “a few precious seconds of The Beatles performing in their leather suits, the earliest known film of The Beatles and never seen before,” provided by Pete Best, the band’s original drummer.

“The result is pretty nutty and provided the video with much needed balance between the sad and the funny,” said Jackson.