By Mark Kennedy

David Letterman joins Bono and The Edge in a new streaming documentary about U2 and one obvious question soon jumps out: What exactly is David Letterman doing here?

The droll, bushy-bearded American comedian is an odd choice to be master of ceremonies for this Disney+ project, unbalancing everything, even the title, "Bono & The Edge: A Sort of Homecoming, with Dave Letterman."

Director Morgan Neville does a fabulous job using new interviews — including brilliant insights by musician Glen Hansard and producer Jimmy Iovine — and melding them with old performances as he explores the band's origins, song creations, highs and lows. But he's torn about whether this is a travel show or a music documentary and neither work well.

The film's spine is a concert by Bono and The Edge at the Ambassador Theatre in Dublin, Ireland, where they unveil some of the ways they've stripped down and reworked their catalogue for the new acoustic-led album "Songs of Surrender," including such songs as "Vertigo" "Bad" and "One."

But Letterman not only distracts, he gets in the way. We watch him wander around Dublin shopping with his clueless, oddball sense of humor, as if the show was about him. "I'm interested in a wheel of cheese. I've never purchased a wheel of cheese," he says.

There's even a weird sequence in which Bono is reduced to drawing a map of Ireland for his guest and unpacking the complex history of Irish-English animus. "Who do I dislike in this?" asks Letterman. The answer is Letterman.

The documentary does a masterful job of giving context to U2's rise, the social, religious and cultural changes taking place in the late 1970s and '80s in Dublin, or as Bono says "as Ireland moves from black and white into color."

There are insights — some small, like that drummer Larry Mullen Jr.'s nickname growing up was "The Jam Jar," and some big, as when Bono reveals tensions within the band regarding his activism — and moments to celebrate, like the band's important Super Bowl halftime show after 9/11.

But then there's Letterman popping up again — visiting polar swimmers or stopping by the recording studio, offering no real insight. Bono and The Edge even write him a goodbye song. Letterman is empty-handed.

Watching the documentary you start to realize how crucial The Edge is, and even get to listen to him sing a few songs and tell the story of how he came up with "Sunday Bloody Sunday" while delivering the riff on a guitar. Bono brings up how key his bandmate is, in a sweet way, live in concert.

"The thing I don't like about Edge is that he doesn't need me. He could be doing all of this — writing, singing, performing, playing, producing — on his own. But he doesn't," Bono says.

"Because it's not as much fun," The Edge replies.

The only thing that ruins this special chemistry is the film's third wheel — the American with the wheel of cheese.

"Bono & The Edge: A Sort of Homecoming, with Dave Letterman," a Disney+ release streaming Friday, is rated TV-14. Running time: 124 minutes. Two and a half stars out of four.

Mark Kennedy is an AP entertainment writer

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