When a mysterious Potions thief begins to plague Hogwarts, Sherlock Holmes and John Watson are called in to investigate. However, the plot takes a surprising turn involving Drarry, sinister pet rabbits, a dodgy pet store supplier, and a divination teacher who does not believe in divination.

Pansy Parkinson spends the last week of her summer before her fourth year at Hogwarts at the Grangers' house. However, she soon begins to fear that she might have had more of an influence on Hermione than she ever intended...