Last year we saw Kevin Macdonald’s One to One, an archive compilation documentary about John Lennon and Yoko Ono’s heady existence in New York in the early 1970s; it took its name from the two charity concerts that they mounted at Madison Square Garden to raise money for children who had been abused at New York’s notorious Willowbrook State School – a scandal to which Lennon had been alerted by watching Geraldo Rivera’s TV exposé. (We have to hope that the box office receipts fully made a difference, but the concert certainly helped change the law to underscore the civil rights of people in children’s homes.)

Now here is the live footage: an immersive split-screen film whose edit was overseen by Sean Ono Lennon. And although no amount of revisionist gallantry can conceal how terrible Yoko Ono’s vocals are, this has a historical fascination as they were Lennon’s only full-length concert performances after the Beatles’ split. And Ono’s performance of the bizarre Open Your Box is certainly arresting: “Open your box, open your box, open your trousers …” There is a heartfelt version of Imagine; a truly apocalyptic rendering of Cold Turkey; and among the old faves are Come Together (after which Lennon says he forgot some of the lyrics: “I’ll have to stop writing these daft words, man, I’m getting old”) and a raunchy Hound Dog (“Elvis I love ya!” he shouts – and perhaps Elvis was aware of this tribute, perhaps not).

For the finale the stage is packed with stars including Stevie Wonder and the inevitable Allen Ginsberg, whose celebrity prestige was mysteriously unassailable. The best track for me though is the first: New York City, John and Yoko’s homage to the city that offered them sanctuary and respite – but would be the site of the terrible catastrophe eight years later.

• Power to the People: John & Yoko Live in NYC is in cinemas on 29 April and 3 May.