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; still, Gacy was 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 and a survivor recalls barely escaping Gacy with his life.

The trial of the most prolific accused killer of his time begins; witness testimony reveals the brutal nature and extent of Gacy's crimes; as the defense focuses on sanity, a jury must decide between sending Gacy 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; a death sentence can't end this story as the specter of more victims has investigators digging again.

Questions linger 40 years later; eight victims are exhumed as authorities work to ID them using DNA; a mother's hunch about her missing son unveils a startling possibility; investigators revisit a property where Gacy worked, looking for bodies.