At once understated and emotionally shattering, Sofia Paoli Thorne’s documentary begins with an image of healing. The camera lingers on the back of a woman, as she applies herbal remedies to her back, flecked with faded scars. This, however, is no casual nighttime routine; now in her 60s, Celsa was once imprisoned in the notorious Emboscada prison, one of the concentration camps that existed during the reign of Paraguayan dictator Alfredo Stroessner.

Before Celsa had set foot inside Emboscada, her mother had already been imprisoned there. The reunion there was bittersweet, and devastating. At the time, Celsa was also pregnant with her son Derlis, who was later born at Emboscada. Her family history is thus inextricably bound to the horrors that the Stroessner’s regime inflicted on dissenting voices. Harrowing testimonies from Celsa and her mother speak of the scorching heat, unimaginable torture and cruel neglect. These vivid stories are heartbreaking, yet Thorne also juxtaposes them with the calm rhythm of Celsa’s daily life, where she carefully tends to her plants and gathers ingredients for herbal cures. Against all odds, the healing has begun.

Like the scars etched on Celsa’s skin, Thorne’s film makes clear that the ghosts of the past can never fully disappear. On the radio, one can hear Mario Abdo Benítez, then president of Paraguay, expressing admiration for Stroessner’s policies. The film’s title, also gestures to the danger of history being erased. The resilient guapo’y tree that used to stand tall in the camp, and was a gathering spot for Emboscada prisoners, was cut down in 2013. Under its branches, Celsa’s comrades would make secret tapes where they documented the camp’s violence and conditions. The film poignantly ends with one of those recordings, voices from the past that seek to rouse the present out of its historical amnesia.

• Guapo’y is on True Story from 14 June.