After around four months, she was allowed to return home to Barcelona for Christmas, but wasn't permitted to go out alone. Somehow - and Mariona doesn't remember how - she managed to escape, but her escape was short lived. Within hours she was bundled into a car with her father and an uncle, and driven back to Madrid.
"We arrived back at the convent at dusk," she recalls. "I refused to go in. They pulled me up the stairs and gave me a sedative to get me inside."
Inside the convent, the other young women were warned against talking to her - the rebel girl who had the nerve to try to run away. She grew intensely lonely, and eventually began refusing food.
Dramatic weight loss resulted in her admission to a psychiatric clinic. There, she says she was given two sessions of electric shock treatment, followed by what was called "insulin coma therapy".
Mariona says she was injected with insulin to induce deep hypoglycemia - a coma-like state caused by low blood sugar. It was believed this could reduce psychotic or schizophrenic symptoms, and somehow "re-set" a patient's brain.
It was a "therapy" that was being discontinued in many countries for one simple reason: it could be lethal.
Mariona received an insulin injection in the mornings. Later she'd be brought out of the coma and made to eat. Mentally, she began to shut down.
"Everyday, I was more dazed. I started saying things like, 'I hurt my parents,'" she says.
"I entered this process of submission and acceptance."
Mariona believes the forced, intravenous "treatment" with insulin irreparably damaged her memory. Suspecting it was causing her to forget things, she began keeping a diary. More than five decades later, this faded, paper document from 1971 would inform Marina's documentary about her mother's experience.
Doctors believed the "treatment" would help Mariona gain weight - but that wasn't happening.
"One day, the psychiatrist decided it was better to try tying me to the bed until I ate."
Mariona's despair became so unbearable, she says she thought about taking her own life. Then the psychiatrist gave her a target weight of 40kg (6st 4lb). If she achieved that, they promised she'd be released from the clinic.