Ptolemy Tompkins was eight in 1970 when his father, hippie visionary Peter Tompkins, invited his mistress to join the family
One night in autumn 1970, when I was eight years old, my father stuck his head in my room and told me he had someone he wanted me to meet. "This is Betty," he said, gesturing toward a blondish woman a little younger than my parents, dressed all in black and looking inexplicably interested in making my acquaintance. "She's come all the way from New York to help you with your poetry."

At the school I attended, just outside Washington DC, my teacher had recently begun reading poems out loud to our class and encouraging us to write our own. I had tried my hand at the craft and enjoyed it so much that I had started sharing my efforts with my parents. Keen as I was about this new pursuit, there was something distinctly fishy about my father going to the trouble of travelling to New York just to secure me a writing coach. As it turned out, Betty was my father's new love and he had really brought her home to meet my mother and to be on hand at our dinner table when my father made a very important announcement - of which Betty herself had no previous knowledge.

"From now on," my father revealed to us that evening, "Betty is to be an integral part of this family. We are about to embark on a unique and potentially very important adventure - one that will change all of us in radical and hopefully very beneficial ways." My father's "great plan", as he outlined it to my astonished mother, involved leaving behind the suffocating blanket of prohibitions and inhibitions that characterised life in twentieth-century America, and entering into a way of being in which absolute freedom of mind, body and spirit was the first and greatest goal. "We must become free," he explained, "and in order to become free we must learn to listen to our true desires and to follow those desires to their end. None of us knows how to do that anymore, because we've been trained to think and act like corpses - dead bodies moving about in a world of shit. From here on out, I've decided to start acting like someone who is alive, not dead, and part of doing that means loving both you and Betty equally, without jealousy and without reservation."

I was too young to take in what was going on in anything but the most vague and impressionistic way. I know that my father said something very much like this because my mother told me about it years later. Just as I know from her that throughout the dinner the strange new woman looked at once pleased and embarrassed to be there, and that my father's statement shocked her almost as much as my mother.

Though his fame as a writer on New Age topics still lay in the future, my father was already well established in the eyes of my mother and I as a figure of prophetic and all but unimpeachable authority. In their 20 years of marriage, my mother had little experience of rebelling against my father, and he must have known it would be a matter of time until she came around. After a week or so of outraged absence, my mother returned to our house ready to set about the "new adventure" of living in the enlightened and jealousy-free manner my father was so anxious to sell her.

From early on, Betty was billed as a sort of intellectual assistant - someone to help my father wrestle with the gargantuan ideas that it was his job to bring to the attention of the unknowing outside world. "It's not just a sex thing," I would hear my father explain mysteriously to my mother. "It's about work! Betty understands my ideas and wants to help me get them down on paper and out into the world. You know my ideas don't always interest you. I had hoped you would be happy that I'd found someone who can fill that need in my life." "I am happy," my mother would reply. "I can understand if you've decided you need to have someone smarter in your life. I'm the first to admit how dumb I am, but why do you need me too in that case?"

"I'm not saying you're dumb!" my father would return. "I'm saying, I'm showing you that just because a person comes across something good in life - a treasure he realises he doesn't want to do without - it doesn't mean he has to turn around and say goodbye to everything else in his life that's good as well. Betty and I get along, we understand one another. That doesn't mean that I suddenly don't understand you, and love you, as I have for all these years. For me to say that would be to give in to what the world out there - the world of Mr Nixon and Mr Agnew and the creepy, hypocritical bastards who run this country - wants me to say. Screw growth, screw possibility. Don't do as you like or be as you like, be as we tell you to be. Or else. Can't you see why it is I don't want any part of that? Why it is I want to rescue you from that sort of poisonous self-denial, even if it means turning this household upside down?"

Despite such elaborate rhetorical fireworks, my father had a rough time keeping the great plan on track for very long. Master at converting others to his way of thinking that he was, he had at last come up with a bill of goods that not even my mother would accept. Nor did my father have much better a time persuading me of the validity of the project. The longer Betty stayed, the more I wondered how he could have possibly got the idea that we needed her around. There was, in my eyes, something peculiar in everything she did - something untrustworthy. Small-minded or not, neither my mother nor I could seem to go through a day without calling into question the idea that this woman belonged among us. What on earth was there about her that my father needed so desperately?

At the end of the first year of Betty's presence, after countless arguments and reconciliations, plane flights, sessions with scientologists and various other urgent attempts to escape the bourgeois world for good, my parents divorced. The great plan, whether from a flaw in its construction or the all-too-human foibles of the three who had enlisted to bring it to fruition, was not to play out as my father had hoped. Yet even after the divorce - partly for my sake and partly because she couldn't really bear to leave my father - my mother continued to live at our house. My mother remained very much at home there, and, after a while, the wringing of hands, the daily emotional alarms, died down. My father had, in a way, got what he wanted.

Yet in another sense he had failed completely - and this was clear even to me. My mother had indeed re-made herself along the lines of the superhuman model my father had placed before her, but ultimately the results were quite different from what he had originally wanted. Though still at least in theory part of the great plan, and as close to me as she ever had been, my mother was at the same time a thousand miles away. She was "above the heaviness", as she would say, using the phrases my father had taught her to feel so at home with; "free from the garbage of the old attachment." But she was also dead: dead to the promises of freedom and possibility that my father kept throwing around, and dead as well to my father himself, and the old love that she had felt for him. "Sometimes I look at your father," she would say to me, "and I don't know what I'm seeing. It's as if he's a total stranger. I just can't believe all those years we spent together. It's as though it were a different life." To be precise, it was the old life. And like it or not, my parents had left it behind for good.

'Paradise Fever: Dispatches from the Dawn of the New Age', by Ptolemy Tompkins, Bloomsbury, pounds 12.99