There's a hidden 4th world about to explode: the world of slums. These sub-populations survive in the direst of circumstances, excluded from mainstream society. What's more they're growing at an alarming rate. In 2007 1 billion people lived in slums, by 2030 it will be 2 billion. This eye-opening investigation brings us inside the world's shanty towns and explores the reasons for their rapid expansion and the consequences if this mass of deprived humanity is ignored.

In an endless undulating landscape of rubbish hundreds of people fight with the crows over the edible contents of this man-made landscape. Hungry children stare dolefully from the door of a shanty shack. A man labours under the weight of a bucket of water on a shanty street of mud, crowded by low dwellings. These, by now familiar images, are replicated around the world. But what are the stories behind them? Stretching across three continents, from the slums of Nairobi, Guatemala City to Manila, this powerful doc looks past statistics, investigating the reality of one the largest social migrations witnessed in mankind's history.

For Pastor John Makwata, there are two very different sides to Kenya. The first is experienced only by tourists; that is, an unparalleled Kenyan hospitality and spectacular landscapes ranging from rainforests, mountains and beaches. The second involves a life shaped by poverty and crime. "Only three things happen to a person if they cannot go to school. Either you become a chang'aa brewer, a thug, a thief or a drug baron."

Living in exceptionally harsh conditions; lacking basic daily necessities; denied access to education or legitimate employment: crime becomes the only means of survival. As a result governments are able to dismiss the plight of slum-dwellers, deeming them a lost cause beyond repair; after all, "it is only with a critical mass of informed citizens ... that politicians are both disciplined and permitted to adopt more intelligent policies."

For Selma from Guatemala City, sexual exploitation in particular has defined her experiences of the slums. Her accounts are truly harrowing: sexually assaulted with the consent of her own mother when she was 8, sold into the sex trade aged 9, and raped by her father whilst seeking asylum. Her story is even more unsettling once we are given the supporting statistics; according to UNICEF, "1.8 million children – mostly girls – enter the multi-billion dollar sex trade every year". In the slums, crime comes in many forms.

Poverty, unemployment, illiteracy, crime, socio-political marginalization; all of these factors contribute to the ever-worsening state of slums across the world. This informative and at times hard to watch doc gets right to the heart of the matter, revealing both the struggle of life for those within the slums and the wider implications that their mammoth expansion will have on our world.