Mackenzie Phillips was 10 years old when her father taught her how to roll a joint. She had her first taste of cocaine at age 11. At 14, she landed a role in the film American Graffiti , and one week after her 18th birthday, she was arrested for the first time.

Now, at 49, the daughter of rock legend John Phillips—leader of the Mamas & the Papas —is revealing the most shocking part of her past for the first time. It's a secret Mackenzie says she's kept hidden for more than 30 years.

"You often hear on television people say, 'It's shocking,'" Oprah says. "And, in this case, I would have to say it really is."

In her memoir, High on Arrival , Mackenzie shares details of her dysfunctional childhood and drug addiction, all of which lead up to a stunning revelation on page 108.

Frightened and vulnerable, Mackenzie reads the page aloud: "I woke up that night from a blackout to find myself having sex with my own father. I don't remember how it started or, thankfully, how it ended. … Was it the first time? Had this happened before? I didn't know and I still don't. All I can say is that it was the first time I was aware of it. For a moment I was in my body, in that horrible truth, and then I slid back into a blackout. Your father is supposed to protect you. Your father is supposed to protect you, not f*** you."