Transcript for Scientology Leader David Miscavige's Father on Son's Childhood: Part 1
Tonight on "20/20" -- Usually when you leave something you don't have an organization attacking you. Following you. Reporter: You mean something like this? Having a stranger lurk around a quiet residential street in west Allis, Wisconsin. The people called the cops, and said, listen, there's a guy looking in a house. We think he may be a drug dealer. Reporter: The police are called in, only to make an eye-popping discovery. The mystery man's rented SUV looks like an arsenal on wheels, loaded with hand guns, rifles, ammo, a stun gun, high zoom camera, and a satellite computer. It sounds like this guy's going to war. It sounds like he's a hitman, doesn't it? Reporter: That man, Dwayne Powell, swears he's not a hit man. Instead he says he's a $10,000 a week private eye, with a very famous client. The company I work for is huge. Reporte so why are they spying on this 80-year-old man, who plays the horn for a living? What kind of super dark secret world is this you've got yourself involved in? I have never met a more competent, intelligent, tolerant, compassionate being. A story that affects every scientologist. Reporter: Scientology. Powell says the church is his client. And his target? The estranged father of David miscavige, the church's all-powerful, wildly controversial leader. When snb en -- somebody enrolls, consider he's in for the duration. Reporter: Tonight, only on "20/20," the explosive, tell-all book that's causing a he said/they said between one man and the church he left. It was an escape. You think you can just walk out? No. It's not that he wrote a book, he wrote a false story. Reporter: A deafening crack in scientology's royal family. David was backstage literally tearing me apart verbally. Cursing, yelling, screaming at me. Reporter: When he's screaming at you, do you ever think, "I changed this guy's diapers"? How ex-members depict "Gold base," the church's mountainside paradise, scientology-style. Gated, with tight security. They say your mail is checked. That your phone calls are monitored. Is any of this true? Well, some of that is true, but that doesn't make it a prison. Reporter: Tonight, the story the church doesn't want you to hear. I raised him, and to come to this, what the hell is this? This is nuts. Reporter: "A father's story." Good evening. I'm Elizabeth vargas. And I'm David Muir. Right here tonight, the ABC news exclusive. As a father now faces off against his very own son. The powerful leader of the church of scientology. David miscavige. And now the subject of a new book titled "Ruthless." The publishers told us, officials from the church asked them not to release it. What's in it? Dan Harris is about to find out. Reporter: You have written a whole book about your son, and you've called the book "Ruthless." Yeah. Reporter: That's a pretty damning charge to level against your own child. Right. He wasn't always that way. When he was a kid, I am telling you, he was a lovable kid. Reporter: Ron miscavige says long before his son became the almighty leader of one of the most controversial new religions on the planet -- How much must one do to call himself a scientologist? Reporter: Before all of those speeches to cheering crowds of believers, and before all that elbow-rubbing with celebrity scientologists like Tom cruise, John travolta and Kirstie alley, David was just a regular kid growing up in this middle class neighborhood in wilingboro, New Jersey. They had four kids over there, Phyllis and gill had two girls. Reporter: Aluminum siding, public swimming pools, children bicycling in the street. Ron, a salesman and aspiring musician, is raising four kids with his wife, Loretta. The oldest, Ronnie, David and his twin sister Denise, and their younger sister Lori. So you spent 12 years right here on this street. 12 years right here, yeah. Reporter: Ron says young David was a strong student, with an even stronger will. David is not a big kid. Not at all. Reporter: And yet he was getting into fights. He's a tough kid. I mean, for his size. He's like a stick of dynamite, you know? Reporter: In your book, you described him having a habit of saying not-so-kind things about other people, even as a boy. Yeah. Reporter: It seems to me, from Reading your book, to you, in hindsight, that's a bit of a red flag. It was a bit of red flag, in hindsight. Reporter: But, at home, it's not as if Ron himself is receiving world's greatest dad coffee mugs on father's day. Marriage-wise, we didn't have a great marriage at all. We had strife, and there was some domestic abuse, which I don't ever feel good about, and I don't think you can make excuses for that, no matter what or how much time goes by. Reporter: When you, when you say domestic abuse, what do you mean? I'd strike her. I'd hit her in the arm, or something like that. Reporter: In front of the kids? And in front of the kids, yeah. Reporter: Ron's mea culpas notwithstanding, the church says his acts of domestic violence are much more serious and more frequent than he admits. In fact, the church says the book, co-written by another former church member who is now a fierce critic of church management, is filled with half-truths and outright lies. It's, in my view, a literary forgery. Reporter: The church rarely grants on-camera interviews. But it is taking Ron's book so seriously that it dispatched attorney Monique Yingling to discredit the author. What has David miscavige's response been to this book? Well, I think he's -- on a personal level I think he's -- he's probably very, very sad that his father would do this. There seems to be no explanation except that his father is trying to make a buck off of his name. Reporter: This unusual family history and subsequent family feud was set in motion in 1968, when Ron first hears the word scientology at a business meeting. What was it about the word scientology that got you so interested? I don't know. But it did. All I heard was scientology, and I thought, what is that? Reporter: Ron soon learns that scientology is a new religion founded by the science fiction writer, L. Ron Hubbard. There are certain evils in society which definitely should cease, and we're taking responsibility for them. So I went to a place where there was a guy who was teaching scientology. He would do drills to teach you better communication. Reporter: So it was useful for you, I would imagine, as a salesman. Yeah. Reporter: And also as a guy who was in a marriage that had a lot of arguments involved. Oh, yeah, yeah. Reporter: And he says scientology works wonders for him. Soon he starts paying for one of the central practices of the faith, a sort of counseling called auditing, that uses a scientology device called an e-meter. Look, I didn't know what I was looking for, but I knew I was looking for something. And when I got into scientology, I felt that I had found what I was looking for, which did have a lot of answers to life, on a basic level. Reporter: So many answers, so many life-changing benefits, Ron feels a duty as a parent to introduce his son David to scientology as well. He hopes that somehow the auditing can help with his son's biggest problem, a nasty and recurring case of asthma. He would get severe attacks. Reporter: These must've been terrifying episodes for you, as a dad. That's putting it mildly. Reporter: And so it comes to pass that in 1969, at the tender age of 9, David miscavige has his first auditing session. About 45 minutes later, David walks out, smiling, bright. Reporter: And in that moment, the future is born. A future of fame and power as David miscavige rises to the highest levels of scientology. And a future of turmoil and pain as his family life erupts into a civil war. So you think that was the key turning point in his whole life? I know it.
This transcript has been automatically generated and may not be 100% accurate.