In an odd coincidence the cartoon animator who created Scooby-Doo and Yvonne DeCarlo the actress who played Lily Munster were found dead together in an haunted mansion. The cartoon amateur detectives were called to the haunted mansion to investigate the circumstances of their creator's death. After questioning Herman, Eddie,Grandpa and Marilyn, Super-Sleuths from Scooby-Doo couldn't come up with a clue without their canine clue-sniffing comrade, the recently late Scoob.
It's possible that other distractions slowed the usually swift crew who always solved crimes in 30 minutes and that's not counting commercials. Fred blamed Daphne and Velma for being "off task" in Daphne's trailer. Shaggy Rogers said: "Nooo ccccomment, dddude" amid a suspicious cloud of sweet-smelling smoke.
Herman too distraught to talk after Lily's passing sat alone in a prescription drug-induced stupor. Grandpa who died years ago was very little help. Eddie and Marilyn (Pat Priest) were caught "in flagrante delicto" in one of the many upstairs bedrooms. Members of the Adams family were on the list of usual suspects but all had reasonable alibis except for "Thing" who claimed that he never left his box and his hand was involved with himself at the time of death.
To the shock of all involved Scooby-Doo appeared on the scene (He hadn't died. He escaped from the local ASPCA just before the euthanasia) He, of course, cracked the case by stumbling over a figure hiding in a dark hallway. The culprit was none other than Beverly Owen, the first Marilyn who had never forgiven Pat Priestly for replacing her in subsequent seasons of the Munsters and had been driven to a homicidal streak that her love obsession the creator of Scooby-Doo had left her for Lily Munster!