On July 18, 1969, Senator Kennedy drove away from a party on the island of Martha's Vineyard with an attractive young woman named Mary Jo Kopechne. In what he later described as an accident, Kennedy made an unusual turn and then drove his vehicle off a bridge connecting Chappaquiddick Island to Martha's Vineyard. The car landed in a pond, where Kopechne then drowned. Kennedy did not report the incident to the police until the next morning, and his statement then was implausible to many Americans.

This death became a national scandal and hurt Kennedy's image. He entered a plea of guilty to a charge of leaving the scene of an accident after causing injury, but did not serve any time in jail in a sentence criticized for its leniency towards a member of a prominent family. In an additional criticized decision, no autopsy was performed on Kopechne despite the presence of illegality and unanswered questions about her death.