It's Ben Baxter's 11th birthday, and he is particularly thrilled by one of his presents. An aunt has given him The Junior Detective Manual by Ricky Barton. He dips into it and decides he would definitely like to become a detective. His pal Johnny agrees to help. They decide: what could be easier than becoming store detectives?

They enter a large store and start looking for possible thieves, catching the eye of the resident store detective who is convinced they are up to no good. Luckily, after a ride home in a police car and some friendly advice from a police officer, they have much greater success in solving several puzzling crimes. In this they are helped by Ben's sister Katie, a retired Major, Ben's rather odd uncle, who is an electronics wizard, and especially by his unusual pet Chippy, who certainly helps rescue them from more than a few scrapes.