On Aug. 15, 1914, Frank Lloyd Wright was overseeing the final stages of construction at Midway Gardens, a massive entertainment complex on the South Side of Chicago. John, the second of his six children, was helping out on the project, when his father went out to take a phone call.

When Wright came back, he looked shocked and had to lean on a table to keep from collapsing.

“What’s happened?” John asked.

The call had been from Wisconsin, where Wright had built an estate called Taliesin for himself and Mamah Borthwick, his mistress for the last five years. Her two children from her previous marriage were with her at the time.

“Taliesin is on fire,” Wright said. “Why did I leave them today? What if they’re hurt?”

As he scrambled to return home, Wright learned the full extent of the tragedy.

The fire at Taliesin had been deliberately set, and the arsonist had slaughtered Mamah and the children with an ax before turning to six workers who were also at the home.

Five of the nine victims died that afternoon; another two died soon after, bringing the final death toll to seven. As Paul Hendrickson writes in his new biography, “Plagued by Fire: The Dreams and Furies of Frank Lloyd Wright” (Knopf), out now, that day would haunt the famed architect for the rest of his life.

Today, we remember Frank Lloyd Wright as the greatest of American architects, a visionary who designed landmark buildings such as the Guggenheim Museum on Fifth Avenue. In the early 20th century, though, Wright was one of America’s most scandalous celebrities — and it began with his relationship with Mamah, the first of several affairs that would play out in newspaper headlines across the country.

In 1903, 36-year-old Wright was already recognized as an architectural talent, with several major projects to his credit. He lived in Oak Park, Ill., a fashionable Chicago suburb, where his wife Catherine (known as “Kitty”), had just given birth to their youngest child, Robert. Another Oak Park couple, Edwin and Mamah Cheney, came to Wright’s studio and asked him to design a home for them and their young son, John.

Mamah, then 35, was an attractive, intellectually curious woman, and she took an active interest in Wright’s plans for the house, which would be built less than a 15-minute walk from his own Oak Park home.

Though Mamah and Wright seemed to have a connection, it wasn’t until the end of 1908 that casual observers began noticing something was wrong in the architect’s marriage. One guest, visiting for the holidays, wrote in her diary that she saw in Kitty’s personality “knocks against many stone walls [and] brave pickings up from sloughs of despair.”

By September 1909, Wright was headed to Germany, ostensibly to oversee the publication of a book about his work. But, as he wrote to a client, he was also “deserting my wife and the children for one year, in search of a spiritual adventure.” Mamah, meanwhile, left her son and 3-year-old daughter, Martha, behind with Edwin, to meet up with Wright in New York so they could sail to Europe together.

A reporter for the Chicago Tribune spotted the runaway couple at a hotel in Berlin and turned the affair into a front-page story.

The paper sent someone out to Oak Park to interview Kitty, who insisted that her husband was “honest and sincere” and declared, “My heart is with him now. I feel certain that he will come back.”

Wright did return a little over a year later, with Mamah staying behind in Europe for a bit. Local reporters came looking for a new angle on the story. Though he told them his marriage was “none of the public’s business,” one journalist quoted Kitty as saying, “As you see, we are together again. Our family is reunited.”

But Wright had made it clear he didn’t want to save their marriage, and he was only around until he could get away for good. His escape plan involved building a new house in the woods of rural Wisconsin, near his childhood home, which he called Taliesin, Welsh for “shining brow,” because it was set on the brow of a hill overlooking the Wisconsin River.

Though Kitty refused to grant Wright a divorce, Mamah’s own marriage had been dissolved and she’d returned to using her maiden name, Borthwick. (Since she had abandoned Edwin, he was awarded custody of John and Martha.) She moved into Taliesin in late 1911, spurring another round of condemnation from the media. Once that blew over, Wright set about rebuilding his career, opening a new studio in downtown Chicago and occasionally returning to Wisconsin to spend time with his lover.

On Saturday, Aug. 15, 1914, John and Martha were at Taliesin for a late summer visit. Wright had been with them earlier in the week before heading down to Chicago and his Midway Gardens project.

Mamah and her children had just sat down to lunch on the patio while six workmen were in the dining room on the other side of the house — all of them being served by Julian Carlton.

Carlton had been at Taliesin just six months. He’d come from a poor black family in Alabama and worked his way up to Chicago, from where he eventually landed a job as a servant at Taliesin.

He’d recently given notice, for reasons that remain unknown, though his wife (who was also the cook) said that he’d recently been acting paranoid.

After giving everyone their food, Carlton went back to the patio with a hand ax and swung at the back of Mamah’s skull, piercing through to her forehead.

Taliesin is on fire. Why did I leave them today? - Frank Lloyd Wright, after hearing that his ‘love bungalow’ had been torched while his mistress and her two children were staying there

Next, he attacked 12-year-old John. By then, 8-year-old Martha got up from the table and ran into the house. Carlton pursued her, cutting her three times with the ax before bashing her face in with the blunt end.

After dousing the bodies in gasoline and setting them on fire, he likely torched the door to the workers’ dining room, hoping to kill the men as they fled the flames.

Some, however, escaped by jumping through the windows, rolling on the ground to put out their burning clothes. And then, with the house burning down around him, Carlton ran to the basement, climbing inside a furnace boiler to hide.

By the time the police found him a few hours later, he’d already attempted to kill himself by swallowing hydrochloric acid. Though some speculate that he’d been outraged by a racist insult from one of the workmen, Carlton never revealed his motives before dying in police custody a few months later. Meanwhile, newspapers eagerly shared details of the grisly slayings at the “love bungalow” Wright had built for his mistress, with some insinuating that the tragedy was a just punishment for violating the sanctity of marriage.

Wright got to Taliesin late that evening. With his own home now a smoldering ruin, he went to his sister Jennie, half a mile away. He’d designed the house for her years earlier and had insisted on a grand piano in the living room. That night, it’s said that he went to the piano, which he’d learned to play as a child in Madison, and began to pick out a tune from Bach. Crying as he played, nobody dared interrupt him.

The next day, Wright cut down all the flowers in Mamah’s garden, laying them both under and over her body in a simple coffin.

“It helped a little,” he wrote years later, remembering how he’d brought her to the cemetery with the help of his son John and two cousins.

“Together we lowered the … pine box to the bottom of the new-made grave. Then I asked them to leave me there alone.”

As the sky grew dark, he filled the grave himself.

Wright restored Taliesin from the ashes, making it his own home and later expanding it into a studio where he took on apprentices. In 1925, an electrical fire destroyed the living quarters once more, and so he rebuilt the house a second time. But tragedy wasn’t quite done with Taliesin yet.

On a Saturday afternoon in the spring of 1952, Wright was burning some leaves and grass on the grounds of the Taliesin estate. While he was distracted, the wind changed direction, and the flames suddenly grabbed hold of a nearby building where Wright’s apprentices lived.

Hours later, after firefighters got the blaze under control, one of Wright’s students remembers him inspecting the damage and finding a smoke-damaged Steinway piano he’d installed in the dormitory living room. To the astonishment of onlookers, he once again sat down and played through the turmoil.