Arthur and D.W. feel sorry for Grandma Thora because she does not have cable television, cannot eat any good snacks, and has to keep her teeth in a glass. They plan a surprise party for her, but struggle to make things work and do not want any help planning the party. Fern's mother wants her to host her own slumber party to help Fern expand her social skills, but Fern is very reluctant. Fern's guests – Francine, Muffy, Jenna, Sue Ellen, and Prunella – get bored quickly, and the party is a disaster, but when Francine's bracelet goes missing, Fern uses her detective skills learned from Sherlock Holmes to solve the mystery.