Gary Webb, an investigative reporter for the humble San Jose Mercury News, broke one of the biggest national news stories of 1996 with his three-part series “Dark Alliance,” documenting the CIA’s involvement with Nicaraguan drug dealers who had flooded Los Angeles with crack and funneled money to the Contras. In the popular imagination this mutated into a CIA plot to addict black Americans, and the mainstream media came after Webb, who was drummed out of his job and ultimately committed suicide. This carefully written drama, based on a nonfiction book by Nick Schou, notes the flaws in “Dark Alliance” but also exposes the pettiness of Webb’s national competitors (especially at the Los Angeles Times), who preferred to poke holes in his reporting rather than to follow up on his charges. Director Michael Cuesta, a veteran of HBO’s Homeland, imparts enough paranoid-thriller ambience to propel the complex story; with Mary Elizabeth Winstead, Rosemarie DeWitt, Barry Pepper, Michael Sheen, and Jeremy Renner in a strong performance as the hounded journalist.