At exactly 11:15, the front door of a council flat in Brixton opened. Two women stepped out on to a quiet residential street.

The younger woman, Rosie, had an awkward gait. Her movement was stiff and clunky, as though she simply wasn't used to walking any distance. In fact, she had spent the past 30 years - her whole life - in captivity.

Now she was ill and needed urgent medical attention.

Born into a “collective”, she was not allowed to see a doctor, had never been allowed outside alone and had been told that if she tried to leave she would spontaneously combust and die.

Worried she might not survive her illness, on 25 October 2013, Rosie and another woman, Josie, sneaked out.

Waiting for them just round the corner were members of an organisation that helps people who have been abused, trafficked or enslaved. Along with the police, they had helped organise the escape.

It soon became apparent that Rosie and 57-year-old Josie weren't the only women who lived in the flat, and when police officers returned they met Aisha - a 69-year-old woman originally from Malaysia. At first she didn't want to leave, but as they talked, she changed her mind.

In the weeks that followed, it became clear how extraordinary their life had been.

All three women seemed extremely frightened, often referring to an all-powerful force called Jackie, which they believed might seek retribution or cause them terrible harm. They were terrified of electricity, which they called “eeee” and seemed anxious that household appliances might blow up or explode.

As they revealed details of their existence and Rosie gradually became more confident, she decided to change her name to Katy, inspired by the lyrics of Katy Perry's song, Roar, which is about a woman overcoming a difficult relationship and finding her voice.

Katy's own story, and everything she had managed to overcome, proved far stranger than anyone could have imagined.