Enoch Powell was, and why the “rivers of blood” speech was important — but for the most part, McQueen and Siddons provide enough information to make sense to those unfamiliar with these events without having characters dump exposition all over each other.As with “Lovers Rock,” the period detail seems precise, and the music choices provide both energy and context. And even with so many characters on trial, the film finds time to include some nugget of empathy or specificity for each one.