“I am a very angry, optimistic woman,” says Israeli human rights lawyer Lea Tsemel toward the end of “Advocate,” a sober, engrossing documentary dedicated to her life and work. Her description isn’t a contradiction in terms, exactly, but it does highlight two traits that are rarely twinned, particularly to positive effect. It’s an apt pairing, however, both for Tsemel herself — after a career spent fighting the justice system with few outright victories to show for it, she retains a dogged faith in the possibility of change — and for the tough-minded, clear-eyed film that directors Rachel Leah Jones and Philippe Bellaïche have made in unsentimental thrall to her.

Following two cases that exemplify Tsemel’s dedication to defending Palestinians charged with terrorist acts, it’s a gripping procedural that intelligently expands into more searching questions over the Israel-Palestine divide and one woman’s precarious, often unpopular role as what...