Truncated, authorized and sanitized for your protection, “Becoming Led Zeppelin” offers the tamest take you’ll ever see on the hedonistic heavy metal band that started it all.

Director Bernard MacMahon agreed to limit himself to what plays as three grandfatherly English squires, interviewed in the study at Downton Abbey in furniture rescued during The Restoration. It’s as stodgy, stiff and bloodless as it is humorless.

“Becoming” is an “authorized biography,” in other words, with only their voices — and that of late, little-interviewed drummer John Bonham — telling their version of their story.

The average Led Zep fan with any acquaintance with their history and “lore” could serve up more sordid, full-blooded and “lived” versions of their career. The testimony of academics and rock journalists who have studied and covered them is sorely missed.

But when you hear how the already-famous-and-well-off-when-they-formed Jimmy Page had them self-finance and self-produce their first two LPs, you get it. They’ve always demanded “control” and they’ve always gotten it.

So no fish stories here. No drugs. No groupies.

But guitarist Page, singer-and-lyricist Robert Plant, avuncular bassist John Paul Jones and the late drummer Bonham, interviewed on audio tape not long before his untimely death at 32, still manage to paint a picture of how they met, their musical chemistry and the alchemy that created their distinct sound — over-simplified as heavy metal blues, with a blast of bombast and the odd reference to Bilbo Baggins’ nemesis.

The “rare footage” here comprises some stuff an enterprising fan could find on Youtube — Page’s earliest TV performance, as a kid singer/guitarist in a skiffle band, etc. We see Plant as a proto-hippy flower child, trying out groups and music styles and fashions before he and early mate Bonham found their way to veteran recording studio session men Page and Jones’ efforts to remake Page’s disentegrating Yardbirds into a new band.

Jones tells the tale of how Who drummer Keith Moon concocted their name and Plant sums up his melodic blues shouter yowling and blues-simplistic lyrical style as aptly as anyone ever has.

He was “finding the best bits of Black music and putting it through the wringer.”

MacMahon, director of the “American Epic” historic music documentary series, barely scrapes the surface of their innovative, experimental and thunderous early recordings. He serves up lots of concert footage — some of it rare — and full performances of several of their earliest songs to flesh out his somewhat superficial portrait. He gets no sense of what those heady days of recording, touring and exploding on the music scene in 1968 and ’69 were like. And as that’s the limited scope of the film — Led Zep up to 1970 — that’s a problem.

Plant’s “It was just far out. I was having a great time” is about as deep as it gets.

This is why you don’t do an “authorized” biography documentary. I was dial hopping the other day and stumbled into 1985’s “The Beach Boys: An American Band,” an equally opaque, dull “band’s own version” of a group’s history. Seeing Al Jardine and others “perform” their narration, walking and talking across a football field, for instance, tells us everything we need to know about what we won’t know after the film is done.

Far better, less authorized dirt-and-drama-and-strife-and-scandal documentaries about that band came out much later.

If Led Zeppelin’s place in the culture outlives them, later films will plumb the depths of their “real” experience of fame, success, sex, drugs and rock’n roll. This is the coffee picture table book version — grandpa approved.

Rating: PG-13, drug references, and smoking

Cast: Jimmy Page, John Paul Jones and Robert Plant, with an archival audio interview with John Bonham.

Credits: Directed by Bernard MacMahon, scripted by Bernard MacMahon and Allison McGourty. A Sony Pictures Classics release.

Running time: 2:01