A curious sound started to cut through the tension in the stairwell of the World Trade Center's South Tower after that building was hit on 9/11.

It was the lilt of a Cornish ballad, and it was coming from Rick Rescorla, head of security at Morgan Stanley, which employed 2,700 people in the building.

Rescorla sang through a bullhorn to calm a fraught exodus, and when he was done with the evacuation, he sealed his fate by re-entering the doomed building to rescue others.

A decade later, Rescorla's tragic story comes to the opera stage with the San Francisco Opera's commission of Christopher Theofanidis' "Heart of a Soldier." The opera stars baritone Thomas Hampson as Rescorla and is directed by Francesca Zambello, with a libretto by Donna Di Novelli. It premieres Saturday and runs through Sept. 30.

The opera explores what it means to be a soldier – of any kind. And it is a potent love story.

Rescorla was a Vietnam veteran, as was his longtime friend Dan J. Hill, whose character will be sung by tenor William Burden. Soprano Melody Moore takes on the role of Rescorla's wife, Susan.

Back in 2005, when opera director Zambello read James B. Stewart's New Yorker story on Rescorla, she was deeply moved.

"After 9/11, as an artist, I wondered, 'How could I speak to that event?' "

In the Rescorla tale, she found her forum.

"The subject matter felt very operatic to me," said Zambello, who received rave reviews for her production of the San Francisco Opera's Ring Cycle this summer.

She quickly brought the idea of a 9/11 opera – based on Stewart's subsequent book "Heart of a Soldier" – to San Francisco Opera general director David Gockley, who was leading the Houston Grand Opera at the time.

Gockley loved the idea, but he didn't commission the work and assemble a team until he made his way to San Francisco.

Although 9/11 looms large in the opera, Zambello cautions that this is not a 9/11 opera. In fact, the attack plays out in the last scene of the three-act opera.

"In this opera, we examine the strong bond between Rick and Dan, where there is a real sense of Greek brotherhood and fraternity," she said. "It's an opera about honoring those who have fallen as soldiers. It addresses the Normandy invasion, Vietnam and 9/11."

This intimate retelling of the Rescorla story will be done with Zambello's trademark spare, elegant direction.

"We've chosen to portray the events in a suggestive and stylized way," Zambello said. This, she said, proved key to the 9/11 scene – where projections form an expressionistic baseline for the singing and acting.

The underlying political significance of Rescorla's story, which suggests that the truest allegiance in soldiering may not be to the military but to the brothers with whom one serves, was not lost on Zambello.

"This opera is about big scenes, big issues and big ideas," she said.

She posits the opera as fitting in with a tradition of 18th and 19th century opera. "Mozart and Beethoven were addressing strong political issues, and that continues in the 20th century with Prokofiev and Shostakovich."

That tradition, Zambello believes, is missing from the opera stage today.

"Everyone today is trying to make an opera based on a movie," she said. "It's important that we grab ahold of opera again to speak to bigger issues that are part of our lives."

However, the political significance of the opera was purely secondary to composer Theofanidis.

"We really wanted to keep the focus on character as much as possible," said Theofanidis, "though this one has the political element floating in the background."

In crafting the music for "Heart of a Soldier," Theofanidis shot for a Puccini-like approach in which the emotion rises close to the surface of the musical language.

As was the case with Zambello, Theofanidis found that Rescorla's journey fired many musical and dramatic images. The result? He ended up writing the music in 1 1/2 years.

Along the way Theofanidis mined music from World War II and the Vietnam era for the score, as is the chronological progression of the opera. Theofanidis was not above letting the music of Jimi Hendrix and others of the late '60s era influence the score. He said this proved key to his composing, especially since some of the drama in Act 1 occurs in the Drang River Valley of Vietnam, where Rescorla teaches his troops to overcome their fear of battle by singing.

Eras and politics were less important to librettist Di Novelli than was discovering what made the characters tick. To connect with Rescorla and Hill, she read extensively about soldiering, including Leo Braudy's study of war and gender "From Chivalry to Terrorism" and Navy SEAL training manuals.

"One of the things I learned about soldiers is that they go toward danger, not away from it," she said.

That explains why Rescorla did what he did on 9/11, Di Novelli said. That fearless paradigm also explains Dan Hill's conversion to Islam in 1972, before departing to do a stint in Afghanistan.

In the final accounting, the arc of all three characters has much more to do with how they conquered fear.

"This opera is really about a loss of innocence," Di Novelli said.

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