Carmen Aristegui, a leading journalist in Mexico, fights government corruption and the rise of fake news with astonishing resilience and courage. Shot over the course of four years, Juliana Fanjul’s intimate documentary portrait captures first-hand the dangers faced by those who dared to speak truth to power under the controversial presidency of Enrique Peña Nieto.

After her report on Peña Nieto’s swanky mansion, which had been paid for by a firm that was awarded the country’s first high-speed rail contract, Aristegui was illegally fired from her radio station, whose news department received 80% of its funding from the government. Unable to find work at other outlets, Aristegui and her staff continued to publish, guerrilla style, through her website and social media channels; they broke heavy-hitting news such as Peña Nieto’s plagiarism scandal and the government’s brazen misuse of public funds.

Aristegui’s unwavering commitment to freedom of expression came with death threats, bullets sent in the mail, and even an office break-in. While these frightening events call to mind the grittiness of a political thriller, Fanjul opts for a sober film-making style, deliberately avoiding sensationalism. The relentless intimidation faced by Aristegui is portrayed so matter-of-factly that it further drives home the staggeringly frequent killing of dissenting voices in Mexico. Opening with an emotional vigil held for murdered journalist Javier Valdez Cárdenas, Radio Silence counters the weight of the tragedy with the outpouring of support from hundreds of mourners. Aristegui might not be supported by those in power, but her popularity among the wider population shows that the desire for justice and transparency can never be doused.

• Radio Silence is available on 20 January on True Story.