The intriguing story behind “Seberg” and the always-interesting Kristen Stewart promised greatness. But this biopic squanders both; it’s a bland period piece with an irritating lack of focus.

Jean Seberg, an American actress, was best known for a French role in Jean-Luc Godard’s 1960 film “Breathless.” By the end of that decade, the bilingual performer was bored of acting and enthralled with activist politics, in particular the Black Panthers.

On a trans-Atlantic flight, Seberg offers to give up her first-class seats for Betty Shabazz, the widow of Malcolm X, and his cousin Hakim Jamal (Anthony Mackie). The interaction leads to an affair between Seberg and Jamal, both married, and to her being surveilled by the FBI and shot at by Jamal’s wife (Zazie Beetz, very briefly).

Director Benedict Andrews (“Una”) slogs along at a languid pace, cutting between Seberg and the FBI men tasked with stalking her as part of the agency’s COINTELPRO program, dedicated to disrupting political dissidence.

Vince Vaughn plays a nastily short-tempered agent (although it’s hard to take him entirely seriously), while Jack O’Connell (“Unbroken”), his partner, is morally troubled by their treatment of the actress — but not enough to stop it.

“Seberg” isn’t helped by its uninspired screenplay. We’re told at the start that the actress was badly burned playing Joan of Arc in 1957’s “Saint Joan,” later unsubtly echoed when she’s warned she’s “playing with fire.”

The government’s treatment of the actress, who died at 40 in an apparent suicide, is ripe for exploration — it’s too bad “Seberg,” despite Stewart’s best efforts, doesn’t do its namesake justice.