Every Father's Day for years, Adrian Grenier would pick up the phone – and call his mom, Karesse.
"Growing up, she was both a mother and father to me," Grenier says in the new issue of PEOPLE, on newsstands now.
But this June 17, the curly-haired actor will also wish a happy Father's Day to John Dunbar – the dad he reunited with in 2001, after being estranged for 18 years, and with whom he has since forged a warm relationship.
How the two men got to this point is the subject of Grenier's documentary A Shot in the Dark, which will debut June 3 on HBO. Made with friend Jonathan Davidson in the summer of 2001 – three years before Grenier, 30, landed his breakout role as Vincent Chase on Entourage – the film follows Grenier's quest to reconnect with a man he had once called "Daddy" but hadn't seen or spoken to since he was 6.
"I didn't necessarily think I needed a father," Grenier tells PEOPLE, "but you start to wonder, 'Who am I? Does a father really matter?' "
"No" was the message Grenier got growing up in New York City with Karesse, an ex-flower child turned real-estate broker who was 25 when she met Dunbar on a commune in upstate New York. "I was teaching yoga and dance to children," Karesse, 56, recalls. "John wrote beautiful poetry. We had a connection."
Soon enough, they had a baby on the way, someone Dunbar hoped would bind him and Karesse together. "When she told me she was pregnant, I wanted to be there," says Dunbar, a 56-year-old special-education teacher who bears little physical resemblance to his blue-eyed son. Karesse saw things differently. "I was pushing John away," she admits. "I knew he wasn't right for me. I told him I wanted to be free."
Discovering love letters from another man in Karesse's luggage, Dunbar walked away. "I didn't know what to do," he recalls.
Growing up, Grenier saw his father, who lived in Ohio, a handful of times. Then, after a contentious trip, the visits stopped.
That connection hit home when, on Father’s Day 2001, Grenier called his grandfather and was shocked to hear Dunbar's voice. Suddenly the reality of making a film about the meaning of fatherhood became clear.
"I'd thought about him all the time," says Dunbar, who agreed to be in Grenier's film as a way to reconnect with his son. "I had some fears he would make me a villain, but I wanted to see him so much."
A few months after that first call, father and son hugged on Dunbar's front lawn. Since that time, they've stayed in touch.
And what of Grenier's question – does a father matter? "My father," Grenier says, "makes me feel safe and complete. Maybe I can do that for a child someday."
For more on this story, pick up PEOPLE, on newsstands now.