William Shatner doesn’t know what happened between him and his beloved "Star Trek" co-star Leonard Nimoy.

Nimoy, who famously played the pointy-eared half-human, half-Vulcan officer Mr. Spock in the ‘60s sci-fi series, passed away in 2015 at age 83. His son, Adam Nimoy, announced that the star died of end-stage chronic obstructive pulmonary disease at his Los Angeles home with family at his side.

Shatner, recognized by fans as Capt. Kirk, recently wrote a book titled "Boldly Go: Reflections on a Life of Awe and Wonder." In it, the actor candidly reflected on his decades-long friendship with Nimoy, and how it seemingly went sour.

Shatner admitted to Fox News Digital that he still doesn’t understand what went wrong.

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Leonard Nimoy Mr. Spock William Shatner Captain Kirk

Leonard Nimoy as Mr. Spock and William Shatner as Capt. James T. Kirk in the "Star Trek" episode titled "Spock's Brain," which aired on Sept. 20, 1968. (CBS via Getty Images)

"Not knowing was part of the hurt," the 91-year-old explained. "Leonard and I knew each other for 50 years. We were brothers. He was the brother I never had. And we were in each other’s domain again and again… We were really good friends. And then something... happened. I never found out. But in the last six months of his life, he wouldn’t connect with me. I wrote him. [I told him] I loved him. And I knew he was very sick. He was dying."

Shatner claimed that during the final months of Nimoy's life, he didn’t answer his calls. A reason was never given, which made his loss all the more painful.

But a few years later, Nimoy’s daughter reached out to Shatner.

"She must have heard about how pained I was," said Shatner. "And she said, ‘You know, he loved you.’ And that made me feel a lot better."

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William Shatner said he's grateful to have had a decades-long friendship with his "Star Trek" co-star Leonard Nimoy. (Amanda Edwards/Getty Images)

In 2016, Shatner told The Hollywood Reporter he wondered if Nimoy's refusal to participate in a film he was making prompted the silence.

"I thought [Leonard] was joking at first and treated it as a joke because he sometimes would pretend and say, ‘No, I’m not going to do that’ and then say, ‘Yes,’ so that’s what I thought he did," Shatner said at the time. "But that time he really meant, no. ... I just don’t know, and it is sad, and it is permanent. I don’t know why he stopped talking to me."

A year before his death, Nimoy took to Twitter and announced he had pulmonary disease. Nimoy linked it to smoking, a habit he said he quit 30 years before. In January 2015, a month before he passed, Nimoy tweeted: "Don’t smoke. I did. Wish I never had."

His final public statement on Twitter, made shortly before his death, was thoughtful, yet bittersweet.

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William Shatner Leonard Nimoy Star Trek

Leonard Nimoy, who starred in 1982's "Star Trek II: The Wrath of Khan" with William Shatner, passed away in 2015. He was 83. (CBS via Getty Images)

"A life is like a garden. Perfect moments can be had, but not preserved, except in memory," he wrote, followed by his customary "LLAP" signoff — shorthand for "Live long and prosper," Spock’s catchphrase.

Shatner knows exactly what he would tell his pal if they were sitting down together today.

"Well, I wrote him a note, shortly before he died," said Shatner. "I don’t think he read it. Never got a response from this heartfelt note, but I would say to him what I wrote in the note. ‘Hey, my God, you are my friend. If I’ve done anything wrong, tell me about it because I love you. And I value our friendship. Why don’t you tell me what I did? I won’t do it again.' That opportunity never existed, but that’s what I would say to him."

Most recently, the surviving "Star Trek" cast has endured another devastating loss. Nichelle Nichols, who broke barriers for Black women in Hollywood as communications officer Lt. Uhura, died in August of this year. She was 89.

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Nichelle Nichols as Lt. Nyota Uhura

Nichelle Nichols played Lt. Nyota Uhura in "Star Trek." (CBS via Getty Images)

The original "Star Trek" premiered on NBC on Sept. 8, 1966. Its multicultural, multiracial cast was creator Gene Roddenberry’s message to viewers that in the far-off future — the 23rd century — human diversity would be fully accepted.

During the show’s third season, Nichols’ character and Shatner’s Kirk shared what was described as the first interracial kiss to be broadcast on a U.S. television series. In the episode, "Plato’s Stepchildren," their characters, who always maintained a platonic relationship, were forced into the kiss by aliens who were controlling their actions.

Worried about the reaction from Southern TV viewers, showrunners wanted to film a second take of the scene where the kiss happened off-screen. But Nichols said in her book, "Beyond Uhura: Star Trek and Other Memories," that she and Shatner deliberately flubbed lines to force the original take to be used.

Despite concerns, the episode aired without blowback. In fact, it got the most "fan mail that Paramount had ever gotten on ‘Star Trek’ for one episode," Nichols said in a 2010 interview with the Archive of American Television.

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Nichelle Nichols as Uhura and William Shatner as Capt. James T. Kirk in the "Star Trek" episode "Plato's Stepchildren." The episode aired on Nov. 22, 1968. (CBS via Getty Images)

Shatner said he still has fond memories of bringing that scene to life.

"Well, from my personal point of view, it’s overwrought," he explained. "There was this beautiful woman, and it was written in that I kissed her. And so, I kissed her, and it was a lovely… I enjoyed the experience very much. You’re kissing somebody, and it’s great. We both enjoyed it. And then the fallout of Black and White – she’s a beautiful lady. She was a beautiful lady. And the fact that some television stations in the South didn’t play that episode the first time around – it’s different now."

"So yes, if you’re involved in that world of, there was a step forward there,' I’m with you," he shared. "But from my point of view, it was two actors having a nice afternoon."

Shatner has led a successful decades-long career with hit shows, such as "The Defenders," "T.J. Hooker" and "Boston Legal." But in his book, he reflects on life and death. He also detailed his experience of being the oldest man to ever travel to space at age 90. That journey took place in 2021. The aerial adventure was made possible by billionaire Jeff Bezos' aerospace company Blue Origin. The Amazon empire founder credits "Star Trek" with igniting his interest in space travel.

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Star Trek actor William Shatner (R) gestures to Planet Labs co-founder Chris Boshuize

"Star Trek" actor William Shatner gestures as Planet Labs co-founder Chris Boshuizen looks on at the landing pad of Blue Origin’s New Shepard after they flew into space on Oct. 13, 2021. (Mario Tama/Getty Images)

While aging doesn’t always make it easy, Shatner is determined to continue pursuing his passions.

"My shoulders hurt," he said. "I can’t run like I did. I’m hesitating to go scuba diving – I love scuba diving. I scuba-dived for many, many years in various places. [But] the last time I was scuba diving, I sort of ran out of breath. And that scared me. I don’t know whether I can go scuba diving anymore. So what can I do? Ride horses and work out in the pool. It’s what I do."

The Associated Press contributed to this report.