Jesse Plemons appears for all of seven minutes in Alex Garland’s discombobulating Civil War, but in the tradition of Hollywood’s great bench players, it’s enough time to nearly run away with the whole thing. We encounter him when our traveling war reporters — Lee Smith (Kirsten Dunst, Plemons’s real-life partner), Jessie Cullen (Cailee Spaeny), Joel (Wagner Moura), and Sammy (Stephen McKinley Henderson) — inadvertently stumble onto a farm where militiamen, led by Plemons’s unnamed ultranationalist, are pouring bodies into a mass grave. The film never clarifies what’s up with these guys or what led to this particular moment, but that’s...