John O'Neill, a senior FBI counterterrorism agent who had repeatedly warned his superiors about the threat posed by Osama bin Laden and al-Qaeda, resigned from the bureau in frustration weeks before the 9/11 attacks, only to die when the World Trade Center towers collapsed. He had just taken up a new job as head of security at the Twin Towers when the attacks he had long predicted finally happened, a twist that has been described as reading like a Greek tragedy rather than modern history.

After his death at the age of 49, an astonishing personal life emerged, with O'Neill revealed to have been simultaneously involved with several women in different cities who were unaware of one another until contacted by a journalist writing his obituary. These included his estranged wife Christine, with whom he had two children, girlfriend Valerie James in New York, Mary Lynn Stevens in Washington DC, and Anna DiBattista, who dated him for seven years and believed she was his only partner. DiBattista, speaking exclusively to the Daily Mail, described O'Neill as a charismatic, high-flying figure who took her to FBI functions and introduced her to his family, yet skilfully concealed his other relationships, saying "he was the ultimate spy".