It was 1969, and 8-year-old Amy Grappell thought her house was haunted. She’d seen shadows pass her bedroom door in the dark of night. Once, she was sure she’d heard a ghost; the stairs creaked as if the specter were walking in the hallway. Amy mustered some courage and got out of bed. A strange man was coming out of her mother’s bedroom. Was he real? Their eyes met.

“Are you sleeping with my mother?” the child asked. She meant sleeping in the literal sense—Amy didn’t know what sex was, but the whole thing seemed like a betrayal all the same.

When Amy asked her mother about the man, she was told that she’d been imagining things. “I began to doubt my own sense of reality, which led to a profound sense of instability,” Grappell told me.

It turned out that Amy’s parents, Paul and Deanna, had entered into a group marriage with another married couple, Eleanor and Robert, who lived in their suburban Long Island town. “The men would tiptoe out of their houses after us kids were asleep to switch wives,” Grappell said. “To keep up appearances, the right car had to be at the right house in the morning.”

It was the era of free love, and the couples were struggling with the monotony of marriage. “Their individual marriages were failing, but they found that, together, they were happy,” Grappell said. The couples thought they had found an alternative to divorce—“a brave new world that would pave the way for how couples would live in the future.” The four-way affair proved so satisfying, in fact, that the couples agreed to make it official. They entered into a domestic-living experiment that they called the “quadrangle.” “It allowed them to fulfill their emotional and sexual needs while maintaining their marriages and social status,” Amy said.

The new arrangement proved destabilizing for the couples’ kids. For Amy, it was especially deleterious. “When the other family moved into our home, it was like an invasion,” Grappell said. She felt neglected. “My feelings of abandonment and desperation were the enemy of their utopia.” The psychological effects haunted her for years.

Ultimately, Amy found an unconventional way to process the trauma. “I had always wanted to tell this story, but wasn’t sure how,” she said. She decided to make a short documentary that would force her to talk openly with her parents. “I don’t think I had any idea how difficult it would be,” she said. “It was like walking through a minefield of the past for all of us.”

The result, Quadrangle, features separate interviews with Paul and Deanna, who have been estranged for many years. An inspired editing choice has them appearing side by side on the screen, forming a diptych of converging and diverging sentiments. There’s a certain voyeuristic excitement to watching the story of the unconventional relationship unfold through individual memory. But underneath is a palpable darkness—the invisible force of the kids’ suffering, and the eventual dissolution of the relationships.

“My parents strived to create a utopian version of family, but in the end, ego trumped idealism, and the relationships unraveled,” Amy said. Both couples divorced and married their foursome partners.

Amy revealed that the trauma she suffered growing up as a child of a group marriage has made her more conventional in her romantic relationships. She is now married, but “not having a functional family model myself, I chose not to have kids, though I wanted them.”

Amy Grappell is working on an upcoming memoir about her experience.