Sophie Lionnet thought she had found the perfect family to work for when she moved from France as an au pair. The bright and bubbly 20-year-old wanted to improve her English and thought the best way she could do this would be to live and work in London.

Sabrina Kouider and her partner, Ouissem Medouni, seemed like a loving family with a welcoming £900,000 home in the upmarket suburb of Wimbledon. But her dreams of a new life were shattered and became a living nightmare as Sophie was starved, tortured, and forced to sleep in a tiny, dark bedroom.

A heartbreaking note, found after her murder, simply said "Why me? I need help to stop them." Sophie was beaten, whipped with a cable, and left with a broken chest bone and ribs before being drowned in a bath.

Their motive for the murder was bizarre - the pair were driven by an obsession ...