Susan Saladoff, director of "Hot Coffee," says the documentary is an outgrowth of her desire “to make the world a better place.” (MIKE SEGAR/REUTERS)

Stop her if you’ve heard this one:

A woman is buying coffee at the McDonald’s drive-through. While steering with one hand and sipping with the other, she drops her drink on her lap. The coffee burns; she sues McDonald’s and walks away a millionaire. And so begins the age of the frivolous lawsuit.

Susan Saladoff has no patience for that phrase. “What’s your definition of ‘frivolous’? Most people think the McDonald’s coffee case is frivolous, until they learn the truth,” she says.

The true story, like most true stories, is a little more complicated than the better-known version. Stella Liebeck, then 79, spilled the coffee while sitting shotgun in a parked car. The drink had been brewed to 180 degrees. Liebeck suffered burns so severe her doctors worried she might not survive.

“Look, everybody knows coffee is hot,” Saladoff said, leaning across the table at Starbucks. “But nobody expects that if you’re buying coffee through the drive-through and you spill it on yourself, you’re going to need skin grafts.”

Saladoff is the director of “Hot Coffee,” a documentary that uses the notorious McDonald’s case to launch an exploration of justice, or lack thereof, in the United States. The Philadelphia native is an unlikely Sundance breakout, as is her movie, which is essentially an 88-minute takedown of tort reform.

Saladoff first considered a career in politics but shifted gears to law during college at Cornell. “I realized I wanted to make the world a better place,” said Saladoff. “And that maybe I could do that more effectively as an attorney than as a politician.”

After college, Saladoff spent a year in New York, studying for the LSATs by day and working as a singing waitress at a dessert cafe by night. (“Well, I wanted to be a Broadway star,” Saladoff explained, laughing at the memory. “But I wasn’t that good.”)

After graduating from George Washington University Law School, Saladoff turned down a job on Wall Street and went to work as a staff attorney at Trial Lawyers for Public Justice. She practiced law in D.C., Maryland and Oregon, where she now lives with her two daughters.

“I thought that I could move the law in a particular direction one case at a time,” said Saladoff. “But after 25 years, I wanted to move it a little faster than that.”

So she applied her theatrical flair to filmmaking. Even though she “knew almost nothing about the film industry,” Saladoff set out to document the McDonald’s case on camera. Saladoff took the resulting 26-minute short — which included graphic photographs of Liebeck’s burns that Saladoff obtained from her doctor — and showed it around the country in an effort to raise money for the rest of the production. What started as a one-year sabbatical from her practice turned into a three-year project, culminating in the documentary’s premiere at Sundance and purchase by HBO.

Saladoff is still floored by “Hot Coffee’s” reception. “The film has taken off in ways that I had dreamed about and believed could happen, but the fact that it’s actually happening is — the word ‘awesome’ is now colloquial, but I am in awe, so therefore it is awesome.”

She brushes her bangs out of her eyes and sits up eagerly in her chair. “I had an idea: I’m going to go make a movie. And then it’s at Sundance. And then HBO buys it and 2 million people are going to watch it. Hello! How great is that?”

Her energy could power the whole street. “I want people to actually take action,” she said. “I want them to be informed about what they’re voting for . . . to be unbiased jurors, to be open-minded. I want people to correct other people when they hear myths.

“I want people to be empowered, to take back our justice system.”

She has been on the road, spreading her power-to-the-people gospel across the country, promoting the project. The film covers plenty of ground, too, from Iraq to Nebraska.

“We spent the day in Hot Coffee, Mississippi,” Saladoff added. “We interviewed everybody who lives there. All nine of them.”

Turns out there is only one store in Hot Coffee, Miss.

It’s called McDonald’s Store.