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"American Masters" spends two hours painting a magnificent portrait of Johnny Carson, giving viewers a glimpse of a man no one really knew during the three decades he was the king of late-night TV.

Almost inadvertently, the portrait reveals to viewers that while Jay Leno got "The Tonight Show," David Letterman was his heir.

After Carson retired, he only returned to late-night TV twice. He talked to Letterman on the phone once; he also walked on the "Late Show," got a huge ovation and walked off without saying a word.

It would be his final TV appearance. And when Carson died, Letterman revealed his mentor had been sending him monologue jokes for years.

"Johnny was so proud," says producer Peter Lassally in the documentary. "He would call me and say, 'Did you see? He used three of my jokes last night.' He was like a kid all over again."

Carson didn't have that kind of relationship with his successor. And two decades later, Leno vs. Letterman is still a flashpoint.

"That's tricky because, well, Johnny really loved David Letterman," said Peter Jones, who wrote, directed and produced the documentary. "And Bob Wright, the former head of NBC, talks about how Johnny did express to him his wish that David [would succeed him]. He didn't have anything against Jay Leno, but I think he really took David under his wing."

Leno, meanwhile, was the guy who helped push Carson off the show. As has been widely reported, "Leno's manager planted a story in the press that NBC wanted Leno to replace Carson sooner rather than later. He was furious," Jones said.

Carson announced his retirement shortly thereafter.

According to Leno, "Johnny was always very gracious. There was tension and then, over the years, we got to be friends again."

Leno said they would "talk occasionally," but added, "you always have that odd situation of anytime you invoke Johnny's name: Are you doing it for your own self-aggrandizement?"

It was undoubtedly a tough situation for Leno. And, clearly, Letterman wasn't happy to be passed over.

But he did turn down the chance to replace Leno when it was offered. And he says without bitterness that moving to CBS was "the better outcome for me personally."

It's clear from the interviews that Leno and Letterman had not only different relationships with Carson, but different feelings about him. Both of them talk about what it was like appearing as a guest on "Tonight" for the first time.

"It takes weeks and months for the adrenaline to drain out of your system," Letterman says, "because here it is, it's 'The Tonight Show' and you've gotten laughs and you get to sit down with Johnny Carson. That's the end of the world. That's just it."

Leno, on the other hand, said it was "like your first girlfriend. You're not very good. It's over very quickly. And you just want to do it again."

Letterman, who rarely gives interviews, was clearly emotional when he talked about Carson.

"Johnny Carson coalesced my life," he says. "He gave me something to aim for. Something to emulate. Well, that's not going to happen, but you've got to have a target. He put me together as a person, honestly. He was an American TV star. I don't think there has been a bigger star."

"Johnny Carson: King of Late Night"

The two-hour "American Masters" documentary airs Monday, May 14, at 8 p.m. on PBS/Channel 7.