Florence Welch (left) joins Taylor Swift on her Eras Tour to sing their duet "Florida!!!" in Miami on Friday, Oct. 18, 2024.

"What if I told you I'm a mastermind?"

The song hums beneath "The End of an Era" docuseries on Disney+ as Taylor Swift pulls back the curtain on one of the biggest creative risks of the Eras Tour: secretly building an entirely new chapter around "The Tortured Poets Department" and redesigning the entire set list while the tour was already in motion.

In the second episode, "Magic in the Eras," she explains how the album (written and released mid-tour) demanded more than folding the songs into her acoustic set.

"One thing I've always tried to do is give something to the fans that they didn't expect," Swift narrates. "When we had done the first half of the tour, what the fans didn't know at the time is that I was writing a brand-new album, 'The Tortured Poets Department.'"

The timing left almost no room for error. Swift wrapped the Asian leg of the tour in Singapore on March 9, 2024, and returned to the stage in Paris on May 9. That narrow two month window became the only opportunity to design a new era from scratch and quietly.

"We build this top-secret rehearsal facility for us to lock in the set list," she narrates. "Like I would kind of do surgical tweakings to each song and be like, 'OK, these songs, but cut out the bridge and the second verse of this.'"

The changes were extensive. Eras were reordered. Songs were cut. Swift, her dancers, her band and the production crew had to relearn the entire show.

"That could have gone so bad," she admits. "There was the danger that we ... it could have felt like we had sort of shoehorned something in, or that, you know, people didn't like the new show as much as they liked the old show."

Swift leaned into the gamble. In Paris, moments before debuting the new section, she huddles with her team before walking to the stage.

"I love having a good secret," she says to the camera before getting into the "cleaning cart," a movable production case where her staff would roll her to the stage.

The undertaking grew more complex when Swift later added "Florida!!!" to the Eras Tour with Florence Welch. The team rehearsed inside Wembley Stadium wearing headphones under the open roof to avoid alerting fans gathered outside.

"Before you know it I'm in a random ballroom in the middle of a hotel with Amanda [Balen] at 9:30 at night," choreographer Mandy Moore says of her assistant choreographer. "Literally a week ago this didn't exist."

The two teach the dancers who practice onstage. They bring in Swift who offers suggestions. And finally, the Florence + the Machine front woman arrives.

"I thought I'd just show up and run around," Welch says before realizing just how cinematic and involved the 3-minute song would be. She admits she's never performed choreography at this scale, never ridden a hydraulic lift.

"When I got there and it was like, 'This is your choreo,' I was like, 'Oh no, no, wait, wait, you have me confused. I don't do this,'" Welch says. "I'm going to f*** this up."

When it came time for the performance on Aug. 20, 2024, the magnitude hit all at once as Welch rose through the stage.

"The feeling of coming up for the first time in that lift, it was kind of landing on Mars," Welch says. "'Cause I'd never seen the stage lit up before, the sound of the crowd."

What struck her most was the contrast between Swift offstage and Swift in command.

"You know it's like you see this cultural moment from the outside and I suddenly was like inside of it," Welch says. "It was wild, but it was really fun and also completely terrifying. Also, Taylor's my friend and I know her as this like, very cozy person, and I came out of that lift and I was like, 'Oh my God, it's f***in' Taylor Swift.'"

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This article originally appeared on Nashville Tennessean: Taylor Swift's secret 'Tortured Poets' plan revealed in Eras docuseries

Reporting by Bryan West, USA TODAY NETWORK / Nashville Tennessean

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