William Colby, the OSS and then CIA operative who rose to be head of the agency at an especially difficult time in the mid-seventies, was—on the evidence of this documentary about him by his son Carl—a man of exceptional integrity and self-control. “The Man Nobody Knew” is marked by similar qualities of straightforwardness, precision and a singular lack of ostentation. As such it shows little sign of the deep emotional undercurrents one might expect in a son’s treatment of his father, but it does present a clear, incisive yet tantalizingly incomplete portrait of a fascinating and complicated man.

Colby is probably best remembered as the CIA chief who publicly spilled what many thought to be too many of the agency’s secrets when grilled by a hostile Congress in the aftermath of the twin debacles of Vietnam and Watergate, and who later died mysteriously while out boating. But as this broader treatment shows, his career was far richer in incident than that. After briefly sketching his early life, the film proceeds to his wartime service in the recently-created Office of Strategic Services, and his segue into the post-war CIA. Special emphasis is given to his time in Vietnam and his contact with Ngo Dinh Diem, America’s chosen anti-Communist instrument in South Vietnam, and Diem’s troublesome brother and sister-in-law. A particularly engrossing element focuses on the debate within the Kennedy administration about whether to support the coup against Diem that toppled and killed him in November, 1963, only weeks before Kennedy himself was assassinated.

Holding to a chronological framework, the documentary then proceeds to Colby’s government service in the later sixties and seventies and his vain efforts to prevent the massive bombing of Vietnam that became a part of American policy during that period. It focuses especially on his tenure as head of the CIA in the period 1973-76 when, under prodding from a hostile Congress, he felt compelled to acknowledge the truthfulness of many charges leveled against the agency—like its illegal surveillance of Americans deemed suspicious by the government. Congressional hearings also revealed his central role in Operation Phoenix, a program to deal with suspected Hanoi agents that resulted in brutality and arbitrary executions. The Ford Administration’s concern that Colby was making too much secret information public resulted in his removal and replacement by George H.W. Bush.

The file footage of Colby at the time of his dismissal, like the other mass of archival material assembled and arranged for the film and accompanied by sober, measured narration, shows that even in the most stressful situations Colby radiated a calm, controlled demeanor. But the interview excerpts the younger Colby includes from a wide array of observers, from political figures and reporters to his own mother, suggests, there was emotional and intellectual turmoil beneath the imperturbable surface. The addition of personal observations about the distance he felt from his father while growing up—and the discussion of the elder Colby’s post-CIA decisions (like his divorce from the woman who’d so faithfully supported him)—also point to the complicated nature of the man, though his son presents this material with the same matter-of-fact attitude with which he covers the broader historical context.

In form and approach “The Man Nobody Knew” certainly breaks no new ground; in its straight chronological structure, its reliance on news footage, narration and talking-head interviews and its sober tone it’s an entirely conventional piece of work. But it’s also a rich examination of one of the lesser Washington actors during a significant period in US history—though given the source, a curiously impersonal one.