Truth is an obstacle that the Thurgood Marshall biopic never overcomes.

It's true that a prejudiced judge in 1941 banned the NAACP lawyer and future Supreme Court Justice from addressing the court during the career-shaping case this movie depicts. Marshall had to channel his defense of a wrongfully accused black man through a white lawyer without criminal trial experience.

Marshall is all about the mouthpiece.

To be clear, Reginald Hudlin's movie isn't another example of white-savior cinema. Josh Gad's portrayal of Sam Friedman sincerely conveys a rookie's incompetence, the clown to a righteous straight man. As with Thurgood's blackness, Sam's Jewish heritage makes him an outcast in Bridgeport, Conn., where racism runs deep as the South. Thurgood will swing back at bigots; Sam takes his lumps wryly.

Yet despite another charismatic turn by Chadwick Boseman as Thurgood, Marshall gradually feels less like his movie. Certainly the screenplay by Jacob and Michael Koskoff leans on Sam's story for character arc and jarring comedic moments; his violent beating followed by a sight gag with a knife, his reactions to a flirty juror. Each Perry Mason courtroom moment goes to Gad.

The case is a high society To Kill a Mockingbird, a black chauffeur (Sterling K. Brown, solid) accused of raping and trying to kill a white socialite (Kate Hudson). Thurgood and Sam's cursory detective work leads to Sam grilling the alleged victim into flashback truth.

Then Hudlin shows practically the same flashback minutes later during Sam's closing argument. A clever editor might splice them into one spellbinding sequence. A cleverer filmmaker would encourage it.

Truth and narrative structure make Thurgood a bystander to history he's shaping, dropping in for pep talks during trial recesses and offering superficial personal details, like wife Buster's (Keesha Sharp) fertility issues. There's a hint that Thurgood could be unfaithful and a crude anecdote showing a sense of humor unless it's true, in which case it's inappropriate for this biopic.

Hudlin's movie means well yet seldom fulfills its purpose. A nightclub in Harlem brings the Marshalls together with poet Langston Hughes and author Zora Neale Hurston for name-check purposes only, not their ideals. Instead of contrasting and comparing Northern and Southern versions of racism we get Odd Couple banter and Sam's muddy new shoes.

Marshall is slightly redeemed by end notes spelling out Thurgood's later legal milestones. We can read about the movies we'd rather see: Thurgood winning the Brown v. Board of Education case in 1954 then becoming the first African-American U.S. Supreme Court Justice. True story: Sam wasn't there.

Contact Steve Persall at spersall@tampabay.com or (727) 893-8365. Follow @StevePersall.