Jerry Lewis hates surprises—and on September 5, 1976, the biggest one of his then more-than-30-year career was waiting in the wings at Las Vegas’s Sahara Hotel. Lewis’s annual M.D.A. Telethon, always an event, was, on this night, already the equivalent of a “Very Special Episode”: Frank Sinatra was performing live instead of remote for the first time since 1953.

Frank did his set and presented Jerry with a couple of donations, including one for $5,000 on behalf of his grandchildren. “Listen,” he told Jerry, “I have a friend who loves what you do every year, and who just wanted to come out. Could you send my friend out, please?”

At the same time in New York, Dean Martin’s six-year-old grandson heard his mother scream. “What’s the matter, Mom?” he asked. Pointing at the TV screen, she said, “It’s your grandfather.”

Deana Martin had been watching the telethon, a family tradition that endured even after the breakup two decades earlier of America’s most successful comedy team, Martin and Lewis. “Uncle Frank’s” friend was in the shadows when he first emerged; Deana couldn’t see his face. “I knew his walk,” she told Vanity Fair in a phone interview. “I got chills. My jaw dropped. I called my sister. She said, ‘I can’t believe this.’”

But there they were, Dean Martin and Jerry Lewis, re-united, embracing, and joking onstage for the first time in 20 years, like it was old times. “Here they are, folks,” beamed Sinatra, mic in his right hand, his left arm draped on Jerry’s shoulder. The standing ovation lasted more than a minute.

In John Ford’s classic The Man Who Shot Liberty Valance, a newspaperman ultimately explains to Jimmy Stewart, “This is the West, sir; when the legend becomes fact, print the legend.” Well, this is Vegas, baby, and the facts are legend.

Martin, the suave crooner, and Lewis, the frantic tummler, first teamed up in 1946. According to Lewis’s memoir, Dean and Me: A Love Story, when they met, Lewis—nine years Martin’s junior—brashly asked his future partner, “You workin’?”

They had a phenomenal 10-year run. They were boffo on radio, television, and in movies and nightclubs. But an insurmountable abyss gradually grew between them—and on July 24, 1956, 10 years to the day of their first appearance as a team, they gave their last nightclub performance at New York’s famed Copacabana (the hottest spot . . . well, you know).

Dean’s surprise appearance at the telethon caught Jerry completely off guard. “You son of a bitch,” he can be heard saying, sotto voce, to Frank in the clip, before joking, “Shoulda been a Jew that did it.” And then Frank bows out, and it’s just Jerry and Dean. “So, how ya been?” Jerry begins. “You know, it seems like we haven’t seen each other for 20 years,” Martin replies. Lewis’s response: “Well, you know, there was all those rumors about our breaking up—and then when I started the show and you weren’t here, I believed it.”

Gary Lewis, Jerry’s son, and the front man of the 1960s A.M.-radio hitmakers Gary Lewis and the Playboys, recalls asking his father what he was thinking when Dean came out onstage. “He told me his mind was going a million miles an hour,” he said in a phone interview. Lewis asked himself, “How am I going to handle this? Am I going to joke with him? Am I going to talk normal?”

What Jerry ultimately came up with was a pitch-perfect in-joke, a poignant callback to their first meeting. To his former partner, he asked, “So, you workin’?”

“They loved each other,” Deana Martin said. “There was something amazing about Dad. He was nice and sweet and kind to everyone, and he would let things go. But when he had it up to here, he’d had it up to here. There was no fight or any yelling. He would just be done.”

And so it was up to Frank Sinatra to stage a reconciliation. “He’s the only person who could have achieved something like that,” stated Mark Rozzano, Jerry Lewis’s manager, in a phone interview. No one in the Sinatra or Martin families was aware of his plan. “[I] was as surprised as everybody else,” Nancy Sinatra said in an e-mail. “My dad really knew how to keep a secret.”

Deana, who has performed on the telethon, found out years later from telethon co-host Ed McMahon that Frank had secreted Dean into Ed’s dressing room. She imagines a scene out of Goodfellas, à la Henry Hill entering the Copa through the kitchen: “I’m sure Dad was in his tuxedo. They picked him up, took him up through the back way, underneath through parking lots—all those exciting, not-so-glamorous ways to get into a showroom.”

Adding to the intrigue, Gary, then age 30, and also in the dark about the impending reunion, was at the telethon that night helping his father, getting him sodas, bringing him production papers. On one such errand, he noticed a dressing room door ajar, and spotted Dean. “I thought, ‘What the heck is going on?,’ . . . but I didn’t say anything to my dad just to be safe,” he said. “He always told me he hates surprises, so I wasn’t going to [be the one to] give him this one.”

Why did Frank do it? Director Peter Bogdanovich was in Vegas on that night, an invited guest (along with Cybill Shepherd) of Sinatra’s to see him perform at Caesars Palace. Bogdanovich, who had profiled Lewis in 1962 for Esquire magazine, was “amazed” and “touched” by the reunion, which he and Shepherd had watched on television in their hotel suite. “After Frank’s show [at Caesars] I told him, ‘That was quite a moment,’” Bogdanovich recalled in a phone interview, “and he said the same thing to me that he said on the air, that he thought it was about time.”

Anything beyond this is speculation, states James Kaplan, author of Frank: The Chairman, the second volume in his definitive biography of Sinatra. Kaplan also collaborated with Lewis on Dean and Me.

“Frank was singing live on that telethon for the first time in 23 years,” Kaplan said in a phone interview. “It was a big deal, and I think he wanted his friend along for support. But he also had a prankish side, Frank did, and I think he thought it would be a fun thing to do. My guess would be that when he proposed it to Dean, Dean decided, ‘What the fuck?,’” a response that Kaplan said “was the essence of his menefreghismo”—an Italian word that roughly translates to “don’t give a damn”—“ personality and his version of an enthusiastic ‘yes.’”

The reunion of Martin and Lewis would make international headlines. But to keep the secret under wraps, the building went into lockdown by Lewis staffers and crew whom Sinatra had recruited to pull off the big reveal. Billy Davis Jr. and Marilyn McCoo, founding members of the 5th Dimension, performed that night at the telethon between their shows at the Riviera Hotel.

“They started herding everybody backstage into this room,” McCoo said in a phone interview. “All we had to do was walk outside and get into our car and we’d be gone.” “But they didn’t want it to leak out [about Dean]," Davis continued.”

Concerns about getting back to the Riviera in time for the second show were mitigated by their understanding of the moment’s importance, Davis added. “We wanted to see Jerry Lewis’s face [when Dean came out],” he said. “We were all watching the monitors around the room. Everybody clapped.”

“As soon as Dean walked out onstage, we were free to go,” added McCoo.

In 1991, the couple would perform with the 5th Dimension for the first time in 17 years. “So these reunions happen,” McCoo said with a laugh.

“I loved Dean,” Gary said. “I met him when I was three. He was always ‘Uncle Dean,’ always smooth and cool. That wasn’t put on at all. I re-introduced myself to him that night in case he didn’t know me, because it was the 70s and I had real long hair and was wearing weird clothes. I said, ‘I’m Gary.’ He said, ‘I know.’ And he called me ‘Pally.’” (That was Dean’s signature term of endearment.)

Deana recalls asking her father about that night. His response? “It was O.K.” “That’s the way my dad was,” she said. “He was downplaying it. He didn’t want to make a big deal out of anything. We were just thrilled because we always loved [Dean and Jerry] together. I had kept in touch with Jerry and with Gary, as we had done a show together at Caesars in 1968 called ‘Chips off the Old Block.’ I don’t know if they were talking all the time, but when my brother Dean Paul passed away, Jerry came to the funeral. He didn’t say anything; he stayed in the back. It was very respectful.“

Dean Paul’s death was the catalyst that brought a measure of reconciliation. “Jerry talked about having the fantasy they would start playing golf together, hanging out; it wasn’t to be,” Kaplan said. “They were show-business giants; separate suns in separate solar systems. They weren’t about to become buds. Show-business friendships are funny. Neither Dean or Jerry wanted to yield enough to be vulnerable enough to come to the other guy and say, ‘Let’s really be friends.’”

But Jerry did memorably surprise Dean at his 72nd birthday with a cake. “Why we broke up I’ll never know,” he proclaimed.

It might not have ended there. In the early 1980s, Bogdanovich came up with the audacious idea to team Dean and Jerry again in a movie, along with Frank, James Stewart, Lee Marvin, and Charles Aznavour. It would be, of course, a Vegas story. The ensemble would portray degenerate gamblers. Dean’s and Jerry’s characters would not speak directly to each other throughout the film until the end, when they would smile at one another.

Frank, Bogdanovich said, was first on board. He offered to recruit Dean, and directed Bogdanovich to call Jerry. “Frank called me and said Dean was in,” Bogdanovich said. “He asked Dean if he would mind working with Jerry. Sinatra quoted him as saying, ‘Oh, what the fuck?’”

That left Jerry, who wanted to hear it from Frank before committing. “He called me back and said, ‘I guess you’re right,’” Bogdanovich said. “He joked, ‘I hope this works out, and if it doesn’t, I’ll be just as happy to see you fall on your face.’”

It didn’t. The project fell apart in the deal stage; Dean died in 1995 at the age of 78.

Forty years since the big reunion, the artists interviewed for this story are still workin’. Gary is on the road most weekends singing such hits as “This Diamond Ring” and “Everybody Loves a Clown.” Deana has a new CD, Swing Street and is readying a centennial celebration of her father’s birthday for 2017. Billy and Marilyn are touring with Up, Up and Away: A Musical Fable, a career retrospective and musical portrait of the 1960s. Bogdanovich’s She’s Funny That Way was released in 2014.

And Jerry, who turned 90 in March, has a new movie coming out in September: Max Rose, his first since Funny Bones in 1995. He tours with his one-man show. A clip of the reunion is a high point, Rozzano said. “It’s something the fans want to watch with him in the room. It’s one of the most iconic moments in television history.”

But rest assured, there will be no more surprises while Jerry is onstage. Which is not to say that many people don’t want to try. “Various celebrities have approached me about wanting to come out [during the show], or to re-unite him with someone he worked with,” Rozzano said. “And I’m forever saying, ‘Good lord, no, absolutely not.”

He paused and sighed. “They all want to be Frank Sinatra.”