“Imperium” (*** out of four) was an imperfect but commanding drama about an idealistic young F.B.I. agent (Daniel Radcliffe) who is assigned to go undercover to infiltrate a white supremacy movement and he must try to conceal his true identity as an agent, find out the next steps of the white supremacists, and try and stay alive. Radcliffe is sensational in a change-of-pace role for him and makes this story tense and compelling even as it strains credibility at times. Sam Trammell is also very strong as a Mr. Rogers-like family man with a double life as a hating supremacist.