Using interviews with many party members, including Kathleen Cleaver, wife of Eldridge, and Baltimore-based leader Steve McCutchen, as well as historians, informants, and law enforcement officers, Nelson, whose previous films include “Freedom Riders” and “Jonestown,” chronicles every phase of the Panther story. It begins in Oakland, where the Black Panther Party for Self Defense organized armed patrols to follow police and prevent brutality, and ends with the party’s ultimate demise in a haze of conflicts and recriminations. The film documents the Panthers’ appeal in detail, from the previously little-heard message of Black pride and empowerment to its fashion sense: natural hair, berets, leather jackets, and sunglasses. When the organization is on the rise, making everyone from California state legislators to national media figures like David Brinkley look like chumps, it’s hard not to cheer for them. And as you watch the leadership forge alliances with the Young Lords and even a group of poor white Appalachian migrants called the Young Patriots, the viewer is tempted to forget history and root for them to grow into the transformational movement they sought to be.