Choreographer Mark Morris was "an adorable child" (his words) when he first heard "Sgt. Pepper's Lonely Hearts Club Band." Jazz pianist and composer Ethan Iverson hadn't been born when it was released.

Decades later, they came together over the Beatles' masterpiece to make something new.

"Pepperland" arrives Saturday at the Northrop in Minneapolis, with 17 dancers, a singer, six musicians (Morris performs exclusively to live music) and Elizabeth Kurtzman's colorful Carnaby Street-smart costumes. Commissioned by the city of Liverpool for the album's 50th anniversary in 2017, the show is a hit, touring the U.K. and the U.S.A. ever since, with more than 70 performances to date and more scheduled into 2021.

Morris has been called "the most successful and influential choreographer alive, and indisputably the most musical." Iverson spent five years as the Mark Morris Dance Group's music director before co-founding the Bad Plus, the internationally renowned jazz power trio that has its roots in the Twin Cities. Seventeen years later, after deciding to leave the band, he found himself working with Morris again.

He's glad to be back with an artist he considers his mentor.

"Before Mark Morris, I was the most nerdy sort of insular jazz guy imaginable," Iverson said. "He showed me the larger world of the arts and human connection. … He's a profound genius. His bag of references is incredibly wide and deep. I couldn't have done [the Bad Plus] without Mark.

"I'll do whatever he wants me to do."

What Morris wanted for "Pepperland" was a 60-minute score that's based on an album just 39 minutes long. Rights agreements placed limits on what they could do.

"The rules were quite strict," Iverson said. "We could only use a certain number of songs [six, to be precise]. We were forbidden from altering any Beatles lyrics. I came up with a really nice song that was fragments of other stuff from the album, but they wouldn't let us use it."

Then again, he mused, "Restrictions are when you get inspired."

Iverson knew Morris' passion for European classical music. "A lot of Mark's greatest ballets are set to that. And 'Sgt. Pepper' itself is full of classical music references. The obvious thing to do with '60s music today is to make it rock out more — double down on a heavy beat, add hip-hop and do all that sort of stuff. I felt the total opposite. I was like, let's do chamber music."

To fill out the score, he wrote new songs built on bits of "Sgt. Pepper" — a trombone line, a chord progression, a guitar lick, the piccolo trumpet solo in "Penny Lane" (which isn't part of "Sgt. Pepper" but was meant to be). He titled them "Adagio," "Allegro," "Scherzo" and "Cadenza," after classical forms. Another song — named "Wilbur Scoville," for the man who invented the way we measure heat in peppers — is a blues.

Iverson created a suite, but not for strings. The "Pepperland" band, led by Iverson on piano, includes Sam Newsome on soprano saxophone, Jacob Garchik on trombone, Colin Fowler on harpsichord and organ, Vinnie Sperrazza on drums and Rob Schwimmer on the strangest of instruments: the theremin.

"Liverpool [officials] wanted something electronic," Iverson said. "I think they meant sampling or a DJ — something horrible. [The theremin] was the very first electronic instrument."

Years ago, he had seen Schwimmer play Bach's "Air on a G String" on the theremin, weaving his hands through the air, touching nothing. "I started to cry," Iverson remembered. "I thought it was the most beautiful thing I ever heard."

"Pepperland" features theremin on "A Day in the Life."

"It stops time," Iverson said. "It's sort of the climax of the show."

Not a Beatles singalong

Morris calls Iverson a "great, great composer/pianist genius." He told CBC Radio, "I didn't just want to put on a record and do a dance. I asked Ethan if he would be interested in composing and arranging these songs so they're recognizable or not, and they're as far away from the actual recording as possible …

"The sonorities and instrumentation are an amazing new take on ['Sgt. Pepper'] that I couldn't have imagined."

Critics love "Pepperland" and audiences adore it — even though, as Morris likes to say, "If you're here for a Beatles sing­along, that's not going to happen."

People who try to chime in on "When I'm Sixty-Four" will trip over the rhythms.

"In between six and four is five," Iverson explains matter-of-factly in the composer's notes. "All three (counts to the bar) are heard beneath the music-hall scuffle."

That's apt, considering the way Iverson and the Bad Plus first captured attention by reinventing such songs as Nirvana's "Smells Like Teen Spirit" and Black Sabbath's "Iron Man."

For "Pepperland," Iverson said he transformed "Sixty-Four" and other familiar Beatles tunes — including "With a Little Help From My Friends," "Within You Without You" and "Penny Lane" — into something "profoundly abstract ... just as abstract as the most abstract Bad Plus deconstructions of pop music.

"Leaving the Bad Plus, it was sort of incredible to be involved immediately in this wonderful, incredibly high-level project where I could use all those tools right out of the gate."

Premiered in May 2017, just a few months before his final performances with the band, this project was a big step in his life, post-Bad Plus.

"I mark 'Pepperland' as my real debut as a composer with a clear style of writing longer works," he said.

His first orchestral composition, "Concerto to Scale," written for the American Composers Orchestra, followed soon after. With Iverson at the piano, it premiered in April 2018 at Carnegie Hall's Zankel Hall. "Bud Powell in the 21st Century," his first suite of big band music, premiered in December 2018 at Italy's Umbria Jazz Festival.

"I'm starting to get commissions for things on a bigger canvas," he said. "At least internally, the success of 'Pepperland' unlocked a creative flow that is really terrific. … I'm very touched with the idea of joy in music now."

Busy weekend for Iverson

Iverson is also writing, continuing his award-winning jazz blog "Do the Math," contributing to the New Yorker magazine and turning in a monthly column for JazzTimes. He has made two acclaimed albums for the German label ECM, one with saxophonist Mark Turner and the other with trumpeter Tom Harrell. He teaches at the New England Conservatory.

And he's maintaining his Twin Cities ties. After playing 18 Christmases at the Dakota with the Bad Plus, as well as several non-Christmas gigs, the Menomonie, Wis.-born pianist wants to stay in touch here. Starting in 2018, he has played several times at Crooners in Fridley.

On Friday, the night before "Pepperland," Iverson will be at the Lexington in St. Paul with Broadway singer Marcy Harriell, drummer Sperrazza and local bassist Erik Fratzke, performing an evening of Burt Bacharach music in honor of chef Jack Riebel. Iverson and Riebel are "pretty close to being family," and Bacharach is Riebel's favorite composer.

"Bacharach wrote hit songs, but they're very detailed and complex," Iverson said. "The cherry on top is the lyrics of Hal David. There's nobody better than Hal David. These songs are incredible." There will be no deconstructing "Do You Know the Way to San Jose?" or any other Bacharach tune. "I transcribed them accurately from the records."

Saturday afternoon, Iverson's wife, boxing journalist and author Sarah Deming, will be at the Red Balloon Bookshop with her new young-adult novel "Gravity." Then on Sunday afternoon, Iverson and Deming will be guests on Mary Mack's "North Star Comedy Hour," a live-recorded variety show at the Eagles Club in south Minneapolis.

"After leaving the Bad Plus, it's been important for me to try to keep a connection with the Twin Cities," he said. "This weekend feels like a nice way to see people, and if people are interested in me, they can see me in a couple of different contexts."

Pamela Espeland is the Artscape columnist at MinnPost.com and blogs at bebopified.com.