Long before the midpoint, “Serenity” suggests rather too strongly that all is either “Lost” (as in the TV show) or simply lost (as in missing in action and lacking in persuasion). Some may find the endgame powerful and moving. The writer-director Steven Knight has written some fine screenplays, including “Dirty Pretty Things,” “Eastern Promises” and the one-man Tom Hardy showcase “Locke,” which he also directed. Knight has also written his share of disappointments, among them the WWII romance “Allied” and the Bradley Cooper vehicle “Burnt,” a portrait of a brilliant but temperamental chef. The atmosphere in “Serenity,” by design, imparts a slightly uneasy and hermetic feeling. In Baker Dill, who sounds like a line of gourmet pickles, Knight has the makings of a compellingly messed-up antihero.