This opening episode of new true crime series Encounters With Evil delves deep into the dark world of those that kill, not for revenge or in a fit of rage - but just for the sheer thrill of it. British serial killer Denis Nilsen - who murdered at least fifteen people - took sexual pleasure from killing and kept the corpses in his home for as long as possible as a twisted form of 'company'.

This episode investigates four cases in which girls and young women are held against their will, often for many years. We explore the brutality, cruelty and control imposed on their victims by some of the world's most twisted jailers, including: Wolfgang Priklopil, who kidnapped 10-year-old Natascha Kampusch, the notorious 'Black Panther' Donald Neilson who killed heiress Lesley Whittle, the monstrous Josef Fritzl who kept his own daughter in an underground dungeon for 24 years, fathering seven children with her, and sexual sadist Ariel Castro who imprisoned three girls as his sex slaves for 10 years.