Agatha Christie understood that a richly detailed cast of characters only deepens a mystery, because it gives the reader more possibilities, theories, and suspicions to savor. Screenwriters Phil Hay and Matt Manfredi have learned this lesson well, and their little chamber piece, ably directed by Karyn Kusama (Girlfight), uses smart, layered dialogue to map out the relationships among eight young people who gather at a remote home to celebrate a couple’s recent wedding. The happy newlyweds (Tammy Blanchard, Michiel Huisman) are involved with some possibly sinister weirdos (John Carroll Lynch, Lindsay Burdge), and the bride’s ex-husband (Logan Marshall-Green), attending the party with great reluctance, begins to wonder if there’s something funny going on. Are his raw feelings toward the happy couple just getting the better of him? Beautifully paced, the movie traces a steady arc from amiability to apocalypse.