From the top of a tree, Pip sees an occupied room in an empty house. Fatty has bought himself a host of new disguises and in addition to pulling a few pranks on Mr Goon, succeeds in getting into the house one night and solving the mystery.
There is not the usual interaction between the characters which exists in other books in the series. The story largely involves Fatty. No doubt children will enjoy it, but it is not one of the best and does not compare to the Invisible Thief, the Strange Messages or the Missing Necklace.