The most surprising thing about Reginal Hudlin's film Marshall — which hits theaters Friday — is how emphatically it's not a biopic. You'd be forgiven for thinking it was, based on that brooding photo of Chadwick Boseman as Thurgood Marshall on the poster. He looks solemn, respectable, challenging, earnest, just as a Supreme Court justice in the making should. And the film starts off making certain promises: We first encounter Marshall in his undershirt, dressing and preparing for his day. Boseman is both vulnerable and attractive in those lovely first moments; as we watch the man don his professional armor, there's every reason to think we'll see more of this — of Thurgood Marshall, civil rights hero in the making, learning to become the force that he became.

This is not that story. Marshall is a procedural, and a good but mighty strange one. The film touches on a couple of Marshall's cases (Lyons v. Oklahoma makes an appearance) but the one it focuses on — The State of Connecticut v. Joseph Spell — isn't one people typically associate with Marshall. For good reason: His co-counsel Sam Friedman (a reluctant Jewish insurance lawyer, played by Josh Gad) argued it. The basic setup resembles the case in To Kill a Mockingbird: A wealthy white woman named Eleanor Strubing (played by Kate Hudson) accused her "Negro chauffeur-butler" of raping her several times and trying to kill her. The defendant, Joseph Spell (Sterling Brown), pleaded innocence. Cinematically speaking, this seems like a perfect opportunity to create a black Atticus Finch.

Instead, the film muzzles its main character. The judge (James Cromwell) rules that only Friedman will be allowed to question witnesses; Marshall may remain present, but he cannot speak. The result is an unusually high-stakes buddy film in which Marshall — who initially sees Friedman as a pliable puppet he can strong-arm into vicarious advocacy — comes to respect his co-counsel. And Friedman, a Jewish skeptic who meets his share of racialized violence (thanks partly to Marshall's incendiary remarks to the press), comes to appreciate Marshall's legal mind and ethical imperative.

The trouble — and I hesitate to call it trouble, because this is a very competent film — is that it shouldn't have been called Marshall, because Marshall himself has no arc. The vulnerable man we see in those opening shots evanesces. Boseman is a twinkly, irreverent delight in this film — he certainly does justice to Thurgood Marshall's sense of humor — but he's also basically a superhero. He's perfect from the moment he puts his suit on: composed, strong, convinced, and right. He already knows it all. He doesn't learn, or change, or grow.

That's kind of a shame. The State of Connecticut v. Joseph Spell took place at a time when Marshall's public identity was more linked to criminal defense than to fighting Jim Crow; it would have been interesting to engage with the growth pangs that must surely have accompanied his transition into the Civil Rights powerhouse he became. There's unfortunately no room for development in this version of Marshall; he's fully formed at 32. The effect (as Alissa Wilkinson argues here) is that structurally, Marshall isn't really the protagonist of this film. Friedman is. He's the character who learns and grows and changes — whose personal life we see more of, whose weaknesses we understand.

There are a few other perplexing choices: For one thing, the real Joseph Spell never denied having relations with Eleanor Strubing (it's pretty central that he lies about this in the movie). For another, the film shoots the various accounts of the encounter between Strubing and Spell as flashbacks. This is a truly disorienting decision: It means we see Eleanor's false accusation acted out as if it really happened, alongside every other version of the story as it unfolds. A procedural is about sifting through the evidence and finding the truth; it's a little disconcerting, therefore, for the camera to honor equally a scene that never happened alongside one that did — particularly since accounts of the attack (as they appeared in newspapers) laced the horror with a potpourri of racist tropes.

Finally, the film introduces a number of really intriguing conundrums — including the possibility that the NAACP, which was in crisis, was "using" Joseph Spell and advising him in ways that benefited the institution more than the man himself — but those questions don't really sharpen or resolve.

The film does elegantly capture the ugly cronyism in the court, the ambient intimidation leveled against those who stood up for men like Spell, and the fraught camaraderie that develops between people with a common enemy. It's gorgeously shot, and often surprisingly funny. Gad does a nice job as Friedman — he handles the comedy lightly and the character rises to unexpected seriousness. As for Boseman, he's so compelling he somehow manages to have insane chemistry with everyone in every room — even Dan Stevens, who plays the bigoted prosecutor.

The truth is, I'd love this film had it been called something else. The Thurgood Marshall presented here is an engagingly foul-mouthed superhero, and the film's ending (which I won't spoil here) forces it into stunning resonance with the present. It's a fascinating and somewhat obscure and laudatory slice of Marshall's professional life, and that's all, and that's fine. But there's room for another Marshall picture — maybe one that goes into his meeting with Malcolm X ("I think we called each other sons of bitches and that was all there was," Marshall said once), or his relationship with Lyndon B. Johnson, or his relationship with his wife Buster (which gets short shrift here), or his disagreements with Martin Luther King, Jr.

"The interesting things that can go in a book, you can't write," Marshall once said. True, but maybe you can film them.