In a candid revelation to PEOPLE, Mackenzie Phillips pleads for understanding towards her father despite unveiling a disturbing truth in her newly released book.

The former child star discloses details of a prolonged incestuous affair with her dad, John Phillips, a renowned musician from the iconic ’60s band the Mamas and the Papas.

At 49 years old, Phillips, who battled through drug addiction, legal troubles, and a failed marriage, bares her soul in the memoir titled High on Arrival.

She recounts a haunting incident that occurred on the eve of her wedding to Jeff Sessler, a member of the Rolling Stones entourage in 1979, while she was already a prominent figure playing a teenage character on the television series One Day at a Time.

Recalling the fateful night, Phillips shares, “On the eve of my wedding, my father showed up, determined to stop it.”

She describes being heavily under the influence of drugs at 19 years old and passing out on her father’s bed, only to wake up to a horrifying reality – engaging in a s**ual act with her own flesh and blood.

Reflecting on her father’s behavior, she remarks, “My father was not a man with boundaries.

He was full of love, and he was sick with drugs.

I woke up that night from a blackout to find myself having s** with my own father.”

Uncertainty clouds her memories as she questions whether such incidents had occurred previously, admitting, “Had this happened before?

I didn’t know.

All I can say is it was the first time I was aware of it.”

The aftermath of this disturbing encounter plunged