Search for the puzzles! Seek the missing items! Solve the crime! Inspector Stilton and his young assistant Nat are called on to solve a crime: the theft of 12 precious items from the city museum.

Each double-page spread shows a richly detailed interior of a shop containing one of the missing items hidden within the illustration. At every place Stilton visits, he is called on to solve another puzzle.

This involves either problem-solving, questions of logic, or hunt-and-find. Young Nat also identifies a third set of visual puzzles to be solved (hunt-and-find).

At the end of the book, the reader learns that the thief has dropped a clue in each place visited. The reader must then return to each double-page spread and find these clues in order to deduce the identity of the thief.