On 17th July, 1969, Kopechne joined several other women who had worked for the Kennedy family at the Edgartown Regatta. She stayed at the Katama Shores Motor Inn on the southern tip of Martha's Vineyard. The following day the women travelled across to Chappaquiddick Island. They were joined by Edward Kennedy and that night they held a party at Lawrence Cottage. At the party was Kennedy, Kopechne, Susan Tannenbaum, Maryellen Lyons, Ann Lyons, Rosemary Keough, Esther Newburgh, Joe Gargan, Paul Markham, Charles Tretter, Raymond La Rosa and John Crimmins.
Kopechne and Edward Kennedy left the party at 11.15pm. Kennedy had offered to take Kopechne back to her hotel. He later explained what happened: "I was unfamiliar with the road and turned onto Dyke Road instead of bearing left on Main Street. After proceeding for approximately a half mile on Dyke Road I descended a hill and came upon a narrow bridge. The car went off the side of the bridge.... The car turned over and sank into the water and landed with the roof resting on the bottom. I attempted to open the door and window of the car but have no recollection of how I got out of the car. I came to the surface and then repeatedly dove down to the car in an attempt to see if the passenger was still in the car. I was unsuccessful in the attempt."