Until Christopher Nolan’s Oppenheimer, I’m not sure I’ve ever seen a three-hour movie that I would describe as moving at a breakneck speed. For a large portion of its running time, that was my main impression of this biopic of the J. Robert Oppenheimer, the theoretical physicist centrally responsible for the development of the nuclear bombs that the U.S. dropped on Japan to bring a ferocious close to World War II. Played by Cillian Murphy, Oppenheimer goes through all the expected paces of a life that merits screen dramatization. There is genius tempered by trauma, and there is charisma scorched by tendencies that are self-destructive but conceivably overcome. Hard-won triumph is countered by a fall from favor, and eventually redemption arrives. A person’s whole existence, contradictions and all, is condensed into a three-act structure.

Nolan enlivens the familiar by attacking his narrative with almost alarming energy. Working from his own screenplay, Nolan kinetically depicts Oppenheimer’s progression from brilliant, anguished student studying abroad to a university professor pushing the boundaries of scientific possibility to the head of the Manhattan Project, intently convinced of the moral justness of defeating the Nazis but uneasy about both his place in military structures and, eventually anyway, the massive power he is centrally responsible for unleashing. The narrative intercuts between different points in time, though not nearly as aggressively as Nolan is usually wont to do. The brisk, bold dynamics are more attributable to the ways in which the writer-director pares scenes to the bone. He repeatedly conveys what a scene needs — narratively, emotionally, thematically — in the most efficient manner imaginable and then moves on. The film is like Oliver Stone’s JFK were it built on historical fact rather than delusional conjecture. Where most epic-length films linger and loll, Oppenheimer roars.

To a degree, Nolan is aware he can do this because he has an aces cast that can fill in whatever gaps he leaves in his haste to get to the next cluster of details. Murphy is phenomenal is the title role, intertwining Oppenheimer’s formidable intellect with his weak failings in interpersonal matters. When the film turns to an acknowledgement of horrible suffering brought down upon humanity by Oppenheimer and his team, Murphy deepens the impact of moments that could otherwise falter, playing either too soft or too garishly insistent, depending on the scene. Up and down the cast, the actors give more than they get, with especially strong work by Emily Blunt, Matt Damon, Florence Pugh, David Krumholtz, and Josh Hartnett. As Lewis Strauss, a public servant who has a complex history with Oppenheimer, Robert Downey Jr. is excellent until the last act, when the script calls for a turn to cartoonish villainy, and he humbly obliges.

The collapse in quality of Downey’s performance in the final stretch of the film is emblematic of a general ill turn for the work. Every attribute of the first two-thirds or so is jettisoned in favor of deadening conventionality. Nolan divides the focus between two governmental exercises in procedural questioning. Everything the film steadfastly avoided previously is now glumly present. Most notably, predictable story turns are presented with the fanfare of supposed suspense, and key details, particularly motivations, are over-explained and tediously repeated. The films that zipped and glided gets stuck in the muck of its concluding statement. Make no mistake, Oppenheimer remains an impressive achievement. It fizzles to disappointment rather than disaster.