Skip the compound. Skip the motorcade footage, the sailboat stamped on the presidential seal, the whole gleaming machine of the myth. There is a quieter Hyannis Port story, and it begins with the sister nobody arranged in the photographs.

Rosemary Kennedy was the third of nine. Younger than Joe Jr. and Jack, older than Kathleen. She was born with an intellectual disability into a family, and an era, that had no framework for her — no vocabulary that fit, no schooling built to hold her, no picture of what a full life might look like. In 1941, a lobotomy left her severely impaired. She spent most of the years that followed in residential care at St. Coletta, in Wisconsin, cut off at first from much of the family. Her siblings came to visit in the decades after.

That is the hard part. It is usually where the telling stops — a cautionary footnote, a shadow at the edge of the family portrait, the name people lower their voices around.

But there was a before, and the before happened here. At the house on the water, Rosemary was one of the kids. She sailed. She swam. She played on the long slope of lawn, "naturally, seamlessly herself," in the words of her nephew Tim Shriver, who has called Hyannis Port "the lab in which Special Olympics grew."

Sit with the word he chose. Not shrine. Not birthplace. A lab — a place where something gets worked out slowly, by hand, over years, in the ordinary company of a girl who swam and sailed and belonged before anyone thought to explain her.

The person who did the working-out was his mother. Eunice Kennedy Shriver took what she had watched on that grass and built it into a movement. It started small and close to home. Camp Shriver, 1962 — and here the geography has to stay honest — ran at the family's place in Maryland, not on the Cape. Out of that backyard camp came the first Special Olympics games, held in Chicago in 1968. The Cape was never the address of the idea. It was one of the things that made the idea possible: a place where a child with a disability was simply a Kennedy in a bathing suit, expected in the boat like everyone else.

The Shrivers never let go of the Cape, and the thread still runs back to the Mid Cape today. This past spring, Eunice's son Anthony Shriver rode in the 27th Best Buddies Challenge, the two-day route from Boston to Hyannis Port on May 29 and 30. Patriots quarterback Drake Maye served as honorary chair. The ride raised more than four million dollars for people with intellectual and developmental disabilities, and it finished the way things finish out here — with a clambake on Craigville Beach, in Centerville. Riders off their bikes, sand in the trays, the harbor doing its slow blue work in the background.

Understand what that ride is retracing. Not a fundraiser dreamed up in a boardroom. A line drawn from a girl on a lawn to a movement that now moves through a hundred countries, with one stubborn Kennedy insistence at the center of it: that she was never the exception to the family. She was the reason the family learned to look.

In 2018, on the fiftieth anniversary, Tim Shriver stood on the same Hyannis Port grass where Rosemary had once played. He called her the first teacher of inclusion. Then he lit a flame on the lawn that runs down to the water — the same lawn, the same slope, the light off the harbor behind it, the place where she had run.

The Best Buddies Challenge rides each spring from Boston to Hyannis Port, finishing with a clambake at Craigville Beach in Centerville.