It all starts with a friendly voice that offers vulnerable women an opportunity when they need it the most - until the penny drops, and they realise their lives will never be the same. In Spain, both a destination and a transit country for victims trafficked for sex into Europe mostly from Latin America and Africa, an elite group of women investigators are fighting international gangs to liberate women from the sinister trap they are caught in. BBC 100 Women followed them as they engage in round-the-clock surveillance operations and raids, with unprecedented access into the heart of a global crime that is often hidden just in the flat next door.