Perhaps the most striking thing about “Last Days in Vietnam,” Rory Kennedy’s eye-opening documentary about the 1975 evacuation of the American Embassy in Saigon, is how calmly it surveys what was once among the angriest topics in American political life. The story is full of emotion and danger, heroism and treachery, but it is told in a mood of rueful retrospect rather than simmering partisan rage. Ms. Kennedy, whose uncle John F. Kennedy expanded American involvement in Vietnam and whose father, Robert F. Kennedy, became one of the ensuing war’s most passionate critics, explores its final episode with an open mind and lively curiosity. There are old clips that have never been widely seen and pieces of information that may surprise many viewers.

Pictures, moving and still, have always been part of the American collective memory of Vietnam. The fall of Saigon conjures up the image of a helicopter on a rooftop as desperate people try to climb aboard. One thing I learned from “Last Days in Vietnam” is that it was not the roof of the embassy, as is sometimes assumed, but of the building where the C.I.A. station chief lived, in another part of the city. What happened at the embassy — and in the waters off the coast of Saigon — was desperate and dramatic and much more complicated.

The Paris Peace Accords of 1973 had provisionally maintained the partition of Vietnam into North and South. As soon as the American forces were gone, the Communist North began to unify the country by force, sweeping quickly through Da Nang and other Southern cities and closing in on Saigon by April of 1975. For tangled reasons that Ms. Kennedy and her interview sources manage to clarify impressively, plans for evacuation were delayed until the 11th hour. Thousands of Vietnamese who had loyally served the American cause and the South Vietnamese government were in imminent danger, and “Last Days in Vietnam” is largely a chronicle of efforts to get them and their families out.