A politician, a children's clown, a party host, an entrepreneur, a father and a husband. These were all roles played by John Wayne Gacy to mask his true nature; that of the world's most notorious serial killer in history.

Like the fictional lead character in Robert Louis Stevenson's novel, Dr Jekyll and Mr Hyde, Gacy somehow managed to live a secret life between the best of good and the worst of evil. By day he was a model citizen. By night he would become overwhelmed by a seething rage that he fused with his sexual attraction to teenage boys.

He abducted these naive, young men off the streets of Chicago and drove them back to his suburban home. There, he bound his victims with handcuffs and gagged them with their own underwear. Then the sexual torture would begin.

It would end with this killer clown slipping a rope around their necks and fatally twisting it like a tourniquet. Gacy buried the bodies in the crawl space beneath his home.

However, Gacy's act began to wear thin and an investigation into a missing youth led police to his front door and down into the crawl space. His double life was exposed to the world and he was convicted of murdering 33 boys and young men.

Using rare and never before seen archive materials, the film helps to paint a portrait of this serial killer from cradle to grave. Through exclusive interviews, Gacy's friends and family rekindle the life of the man in front of this monster, while they struggle to answer the question on everyone's mind; why did he do it?