The Purge: Election Year Frank Grillo, Elizabeth Mitchell, Mykelti Williamson. Directed by James DeMonaco. Rated R. Opens Friday citywide.

With 2013’s The Purge, writer-director James DeMonaco used an intriguing (if somewhat incoherent) sci-fi concept as the backdrop for a mediocre home-invasion thriller, but the movie’s unexpected popularity has pushed him to expand the world in subsequent sequels. The third movie in the series, The Purge: Election Year, is the best yet, with the most extensive consideration of the various implications of the high concept (a future America in which all crime is legal for one 12-hour period each year). Election Year centers on Sen. Charlie Roan (Elizabeth Mitchell), a presidential candidate who promises to get rid of the Purge and is thus targeted by the establishment for elimination during the Purge itself.

Frank Grillo returns from the second movie as former police sergeant Leo Barnes, now head of security for Sen. Roan and once again just about the only competent, trustworthy person around during the Purge. Barnes and Roan find themselves on the run through the streets of Washington, D.C., after Roan is betrayed by a member of her staff, and they eventually team up with a gruff shopkeeper (Mykelti Williamson) and his associates to evade the forces that are after them. Grillo is good at playing the tough guy, and Mitchell makes Roan into a believably inspirational figure, while Williamson gives his somewhat stereotypical character a bit more nuance.

DeMonaco also expands on the social-commentary aspects of the story, with explicit references to the way that the Purge targets the poor and minority populations. While he comes up with a few promising ideas (including the concept of foreigners heading to the U.S. during the Purge for “murder tourism”), he doesn’t really get a chance to explore them, since the movie still has to focus on gun battles and brutal killings. DeMonaco’s budget hasn’t caught up with his higher ambitions, and Election Year mostly features characters running through dark alleys or hiding out in dingy rooms. Eventually, the action becomes monotonous, and it’s not much different from 2014’s previous installment, Anarchy. While the world of the Purge doesn’t exactly provide sophisticated political discourse, it remains promising, and DeMonaco seems to be slowly getting a handle on how to realize its full potential. At this rate, the next sequel might turn out to be a genuinely good movie.