The disappearance of a teenage boy in Chicago sends investigators to the home of contractor John Wayne Gacy. A team of officers track his every move until a chance encounter leads to a horrifying discovery in the crawl space of Gacy's home.

An examination of Gacy's past reveals unsettling evidence that authorities knew he was a sexual predator. However, Gacy was still able to evade law enforcement for more than a decade as he committed his deadly crimes.

Gacy details his murders in a chilling confession as police uncover the scope of his crimes; victims' families tell of their repeated pleas to officers to investigate unusual disappearances; a survivor recalls barely escaping Gacy with his life.

The trial of the most prolific accused killer of his time begins as witness testimony reveals the brutal nature and extent of Gacy's crimes. As the defence focuses on sanity, a jury must decide between sending him to a psych ward or a death chamber.

With his execution date set, Gacy's final moments are fraught with controversy, both for the killer himself and his victims' families. However, a death sentence can't end this as the spectre of more victims has investigators digging again.

Authorities work to ID eight victims using DNA; a mother's hunch about her missing son unveils a startling possibility; investigators revisit a property where Gacy worked, looking for bodies.