Randolph, played with the bullying dynamism that is Baldwin’s finest, strongest note, is traced over the likeness of Robert Moses, a storied and polarizing figure in New York history. Norton places him, plausibly enough, at the center of a tale of large-scale malfeasance and personal vice that suggests a variation on “Chinatown.” Postwar New York real estate takes the place of Depression-era Los Angeles water as the all-consuming obsession at the heart of the story. Lionel is the little guy who sticks his nose where it doesn’t belong and sniffs out the corruption of the big shots — the racism, petty cruelty and sexual depravity underneath the grand conspiracies.

His decency is the axis on which the story turns, and also the movie’s principal flaw. Lionel goes into battle without the armor of cynicism that most movie private eyes before him have worn; instead, he is clad in a righteousness that is ultimately hard to distinguish from vanity. “Motherless Brooklyn” devotes a lot of time to explanation, which may be necessary given the intricacy of the plot, but which turns into a lecture after a while. A fable of power dissipates in a fantasy of rescue. Lionel evolves a little too conveniently from misfit to paladin, from ally of the marginal and oppressed to their would-be savior. He’s a kind of noble answer to the Joker, another beleaguered city dweller who explains his strange behavior with reference to a neurological condition.

The high-mindedness of the movie, its showy conviction that its heart is in the right place, dulls some of its political insights. And its grandiosity undermines the ragged pleasures of the genre. Norton seems to have decided that the best way to celebrate Lionel’s underdog scrappiness was to build a monument in its honor.

Motherless Brooklyn

Rated R. Gunplay and wordplay. Running time: 2 hours 24 minutes.