“Nasrin,” a surreptitiously filmed documentary about the imprisoned Iranian human rights lawyer Nasrin Sotoudeh, offers a strangely cheerful portrait of extreme sacrifice and ongoing suffering.

The uplift is a little unnerving, the bright positivity of Sotoudeh echoed among her supporters (including the dissident filmmaker Jafar Panahi) and clients. One young woman, Narges Hosseini, arrested for protesting Iran’s mandatory head-covering law, smiles calmly as she accepts the possibility of a lengthy prison sentence. Her courage, like that of so many in this film, is breathtaking.

Defending women like Hosseini led, in part, to Sotoudeh’s 2018 arrest and a sentence of 38 years and 148 lashes, according her husband, Reza Khandan. A pocket history of Iran’s volatile record on human rights, along with examples of Sotoudeh’s political work on behalf of women, children and minorities, provide context for her various incarcerations as the director, Jeff Kaufman, compiles secretly captured footage from multiple sources. Interviews with Iranian exiles and other activists enrich his portrait, as do warm moments with Sotoudeh, Khandan and their two children.

Yet this extraordinary woman, seemingly incapable of despair through roughly two decades of struggle, remains elusive. There’s something daunting about this degree of implacable selflessness, and it has a curiously flattening effect on a movie that feels less emotionally complex — less enraged — than it ought to.

By the end, I worried mainly about Sotoudeh’s children, enduring yearslong separations from one or both parents. And when a prison visit showed her son laughing delightedly at his mother through a glass partition while her daughter wept quietly nearby, it felt like the most painfully human moment onscreen.

Nasrin
Not rated. In English and Farsi, with subtitles. Running time: 1 hour 32 minutes. Watch through virtual cinemas.