The original idea was to make a 20-minute short about Mike Francesa and Chris Russo, because after all, how much was there to say, and how many people outside the New York area wanted to hear anyone say it?

“We spent a few months researching the story and came back to them and said, ‘This is not a 20-minute short,’” said Dan Forer, who directed the 30 for 30 documentary, “Mike and the Mad Dog,” whose ESPN premiere is at 8 p.m. Thursday night.

“They asked why, and we showed them all the complexities of the story and they said, ‘OK, let’s do an hour,’ and we were off and running.”

The result is a fast-paced, nostalgic ride for New Yorkers old enough to recall when it began and an education for younger sports fans curious about life in the late 20th century.

Does it break much new ground? Not really. Will non-New Yorkers care? TBD. But for most people who are reading these words, it figures to be appointment viewing.

Forer said the “Mike and the Mad Dog” idea had been percolating at ESPN Films for years before he came aboard.

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“I went to ESPN with three different ideas, which they liked but they didn’t love, and they passed,” he said. “I then asked, ‘Are there any stories that you’ve always wanted to do but you haven’t been able to do?’

“They said for over a decade they always wanted to do ‘Mike and the Mad Dog.’ . . . It was a passion project, a story they always wanted to do but never were able to get done.”

Knowing that Forer had worked with Francesa at CBS Sports in the 1980s, they offered the idea to him, and soon Forer was on board, as was former CBS Sports executive Ted Shaker, as were Francesa and Russo.

Shaker, executive producer of the film, said it documents a program that not only helped launched a new genre of radio, but helped save WFAN itself.

“It was going down the tubes; it was doing poorly,” he said. “They had ‘Imus in the Morning’ and then nothing for the rest of the day, until they got to the Mets in the evening. So they put Mike and Chris together . . . In 10 months they became No. 1 in the market – 10 months! And they went on to do it for 19 years.

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“If they had failed, I don’t know where we’d be. But they didn’t. They succeeded wildly. That’s the story to me.”

Shaker said the story has a powerful emotional and generational pull.

“Because of the medium they’re in, radio, they make it a personal connection with people,” Shaker said. “We kept hearing from people who remembered as kids getting picked up at school or going to practice with their mother or father and their dad would have ‘Mike and the Mad Dog’ on the radio. It becomes a connection to their father or their mother.”

The most serious segment details the episode after the terrorist attacks of Sept. 11, 2001, in which Francesa and Russo were accused of making remarks some deemed offensive to American Jews.

The day before the film premiered at the Tribeca Film Festival in April, Deadspin posted a partial transcript of the “lost” tapes of the Sept. 12, 2001, show, which appeared mostly to vindicate the duo. Forer then updated the film to include audio from that day’s show.

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Both Francesa and Russo endorsed the documentary after watching it for the first time at Tribeca, and it has gotten mostly positive reviews. The big question for a nationally televised show is whether people in Peoria or Boise will have any interest in the ups, downs and petty feuds of a pair of New York sport talkers.

“Our hope is it’s seen as an interesting story, well told, entertaining,” Shaker said.

Said Forer, “We will find out whether it has legs beyond the New York metropolitan area. Our real challenge was telling three stories in one. One story was about their relationship and how at times it was combustible. Another one was sharing what made the show for 19 years so special.

“The third was relaying the impact they had on sports media and how they are two men who gave birth to sports talk radio . . . If anybody enjoys sports talk and wants to know why they’re able to listen to it now, I think they’ll enjoy the film.”