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

Scott Thorson, a young bisexual man raised in foster homes, is introduced to flamboyant entertainment giant Liberace and quickly finds himself in a romantic relationship with the legendary pianist.

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 ...