Brian Yorkey wrote the book for “If/Then,” but he credits his partner in all things musicals, Tom Kitt, with the idea for a show that examines the themes of fate and choice.

“(Tom) was very much thinking about the choices we make and the question of, how did I end up exactly here and what things happened to me that I didn’t have any control over and what was the path and how might it have ended up differently?” Yorkey said recently on a call from California, where he was working on a Netflix series based on the young adult novel “Thirteen Reasons Why.”

“I struggled with the idea early on,” Yorkey continued. “I didn’t know how it became a musical. But then I started thinking about this question of, are we destined to meet our soul mate? And once I realized that was an essential question to the whole story, it made sense as a musical.”

That musical, which ran on Broadway for a year starting in March 2014 with Idina Menzel in the starring role of Elizabeth, wraps its national touring debut at the Fox Theatre Aug. 9-14 with Jackie Burns — Menzel’s understudy on Broadway — in the lead.

The story follows Elizabeth, an urban planner nearing 40, as she moves to New York City for a new start. One friend suggests she abbreviate her name to “Liz” and explore new experiences. Another friend advises her to adopt her college nickname of “Beth” and make professional connections in the city.

Hence, the two paths that Elizabeth’s life could take are then explored throughout the show, with New York City as a central backdrop.

“Tom and I are both New Yorkers, so we wanted to write a little bit of a valentine to New York because that city is so full of possibilities. It really is a place where you can round one corner and meet someone and change your life,” Yorkey said.

While the Broadway show was heralded for the return of Menzel to the stage — and the “Let It Go” superstar revisited the role for the first dates of the national tour last year — Yorkey has effusive praise for Burns.

“There aren’t many mortals who can sing a score written for Idina,” Yorkey said with a laugh. “Jackie nails every note but very much in her own style and her own power. I don’t know how that voice exists coming out of such a petite and lovely woman. She is fearless and opens herself up to us. The emotional experience is immediate. Audiences are so lucky to see her. She went on many times (on Broadway) while Idina was in the process of getting very famous, and every time it felt like the show sailed on.”

Before “If/Then” hit Broadway, Yorkey and Kitt’s searing rock musical, “Next to Normal,” won three Tony Awards in 2009 and the Pulitzer Prize for drama in 2010.

The show that focused on the effects of bipolar disorder was almost universally lauded, but “If/Then” elicited a lukewarm reaction from many New York theater critics. Yorkey understands the difficulty of following a megahit, and also acknowledges that the concept of “If/Then” is ambitious.

A few tweaks were made for the touring production — including a new reprise of a song in the second act — which Yorkey believes make the show better.

“It still requires a certain level of engagement to follow every moment, and not everyone is able to do that,” he said. “Some theatergoers do, some don’t. The thing Tom and I have learned is write the story as best we can write it. The rest is sort of up to the world. I would be lying if I said I didn’t care about reviews, but it doesn’t change the fact that I’m really proud of what this show is and the people who love it, love it. If not as many people love it as ‘Next to Normal,’ I’m OK with that.”