After nine and 1/2 hours of Shoah, Claude Lanzmann's exhaustive and exhausting oral history of the Holocaust, we're left with unforgettable moments.

Like the moment when a farmer who tilled his fields near the Treblinka death camp recalls the screams of Jewish prisoners: ''At first, it was unbearable. Then we got used to it.''

Or the moment when Simon Srebnik, a survivor of the genocidal campaign at Chelmno, returns for a reunion with villagers who profess to be happy about his survival. ''Why do they think this all happened to the Jews?'' Lanzmann asks the villagers through an interpreter. ''Because they were the richest!'' a villager replies. Srebnik winces.

There's the moment when Abraham Bomba, a barber who cut the hair of women bound for the Treblinka gas chamber, breaks down during Lanzmann's inquiries. Lanzmann is persistent: He must know what happened when Bomba's friend, a fellow barber, realized his wife and sister were among the prisoners about to be gassed. ''Don't make me go on, please,'' Bomba implores Lanzmann. But Lanzmann is quietly, implacably firm: ''We must go on.'' So Bomba tries to describe a scene almost too agonizing for mere words.

Later, there's a moment when Franz Suchomal, former SS Unterscharfuhrer at Treblinka, vigorously sings a tune taught to Jewish prisoners at his death camp. He finishes the song, then tells Lanzmann: ''No Jew knows that song today.'' Suchomal smiles as he speaks.

Henrik Gawkowski doesn't smile as he remembers driving the train that brought whole boxcars of Jews to Treblinka. He talks of hearing the moans and shrieks over the sound of his locomotive. He talks of remaining almost constantly drunk to deaden his senses. He talks of trying to warn his disembarking passengers that they were not going to work details, that they were about to be processed by a killing machine. He traces a line across his neck with his index finger. The moment is terrifying.

Such moments are separated by many long minutes and hours during Shoah. (The title, a Hebrew word, means “annihilation.”) No doubt about it: This epic 1985 documentary, 12 years in the making, is punishingly long, rigorously demanding and deliberately repetitious. And yet it remains irresistibly mesmerizing from start to finish, a towering achievement with a cumulative impact that is nothing short of devastating.

This week at the Museum of Fine Arts, Houston, you have the choice of viewing it all in a single day — starting at 10 a.m. Thursday, with a lunch break at intermission — or in two parts at 2 p.m. Saturday and Sunday. Either way, expect to be enthralled and amazed.

Without resorting to documentary footage or period photographs, Lanzmann strives to re-create and re-examine the Holocaust by presenting it through the words of survivors, witnesses, perpetrators and not-so-innocent bystanders. His approach is remarkably effective, and his interviews — some of them recorded with hidden video cameras — are chillingly enlightening.

He juxtaposes the words with jarring images. The lush green fields we see once were the site of mass graves described by death camp survivors. The camera sweeps us down a long country road, forcing us to retrace the route taken by Jews on their way to destruction at Auschwitz. And repeatedly, insistently, there are the trains: belching steam, rattling along tracks, relentlessly moving toward the end of the line.

The device is poetic, but the interviews are prosaic. Lanzmann doesn't want to deal in euphemisms or generalizations. He has the patience to ask specific questions: How big were the crematoriums? How many people died each day in the Warsaw ghetto? Exactly how did the German government pay for the ''resettlement'' of Jews? (A low-level Nazi era bureaucrat recalls buying one-way tickets — at excursion-rate prices — with money confiscated from Jews when they were arrested. That's right: The victims paid for their own trips to the gas chamber.) What was the life expectancy of a Jew who arrived at Treblinka? (Usually, four hours.) How did SS commanders dispose of so many bodies?

And most important of all: Why? Why did the Polish underground refuse to give weapons to the Jews in the Warsaw ghetto? Why did the Allies ignore the pleas of Jewish leaders to launch a special campaign against the Holocaust? Why did people in Germany and Poland deliberately ignore the unmistakable evidence of the monstrous crimes being committed at the death camps?

Why did this all happen to the Jews?

It’s clear that, by the time Lanzmann started work on Shoah, four decades after the end of World War II, many of the Holocaust survivors he interviewed had moved beyond grief, had numbed themselves so they could live with the guilt of living while so many others died. (''If you could lick my heart,'' a survivor tells Lanzmann, ''it would poison you.'') It’s also obvious that other interviewees, for a variety of reasons, preferred not to remember what they had seen or experienced, what they had done or had done to them, and needed to be coaxed, if not coerced, into giving their eyewitness accounts.

Lanzmann would not let anyone forget. And his lasting legacy is an unforgettable film.