S5·E4 Dial M for Dick

Mary invites the Solomons on a murder mystery weekend, thinking that it might prove a fun activity. However, she goes down with food poisoning and retreats upstairs to her room. Downstairs, meanwhile, the Solomons are horrified when the first murder takes place, believing it to be real and, annoyed at the apparent lackadaisical efforts of Scotland Yard detective "Inspector McAffrey", they decide to try to solve the mystery themselves.