One night in the summer of 1981, Scott Thorson went to the home of Eddie Nash, a Los Angeles nightclub owner and drug dealer.

He and the participants in the documentary—former detectives, prosecutors and Scott Thorson, Liberace’s longtime lover, who died Aug. 16—have been living with the case for more than 40 years.

Scott Thorson, the former lover of Liberace and a key witness in the trial for the 1981 killings known as the Wonderland ...