At Harvard, Plimpton was a classmate and close personal friend of Robert F. Kennedy. Plimpton, along with former decathlete Rafer Johnson and American football star Rosey Grier, was credited with helping wrestle Sirhan Sirhan to the floor when Kennedy was assassinated following his victory in the 1968 California Democratic primary at the former Ambassador Hotel in Los Angeles, California. Kennedy died the next day at Good Samaritan Hospital.