Raised in the System,” the first episode of the new season of HBO’s news-magazine series “Vice,” provides an insightful look at juvenile mass incarceration in the United States, managing a tone that’s both grave and encouraging. Our guide is the actor and activist Michael K. Williams, who played Omar on “The Wire.” Williams grew up in the Vanderveer projects, in East Flatbush, Brooklyn, and has been visiting incarcerated loved ones since age seventeen. In 2014, when Williams was appointed the A.C.L.U. ambassador for ending mass incarceration, he told me about some relatives in the system; in the documentary, we meet his nephew Dominic, who at nineteen was sentenced to twenty-five years in prison for second-degree murder, and his cousin Niven, who entered the prison system at the age of fourteen. Felicia Pearson, who played Snoop on “The Wire,” talks about her experience, as a teen-ager, in a maximum-security prison for adults, and we meet youth mentors from Richmond, California, and a juvenile-court judge from Toledo, Ohio, who help at-risk teens and ex-offenders navigate their lives. If you happen to associate Vice with the kind of work that drew the Times media reporter David Carr’s scorn in the documentary “Page One,” from 2011, “Raised in the System” will come as a welcome surprise.