The Three Investigators are Jupiter Jones, Pete Crenshaw, and Bob Andrews, all of whom live in Rocky Beach, a small city on the shore of the Pacific Ocean some miles from Hollywood. Pete Crenshaw is the second investigator and the athletic part of the trio. Bob Andrews does all of the records and research of the group. Jupiter Jones is the stocky first investigator of the group.

As a very small child, Jupiter Jones appeared in a television series about a group of comical children. As an infant he was so fat and comical in appearance, he was known as Baby Fatso and made millions laugh at the way he kept falling over things. This gave him a deep aversion to being laughed at. In order to get himself taken seriously, he studied furiously. From the time he could read, he read everything he could get his hands on, science, psychology, criminology, and many other subjects. Having a good memory, he retained much of what he read, so that in school his teachers found it best to avoid getting into arguments with him about questions of fact. They found themselves proved wrong too often.

The three boys make their headquarters in an old trailer in the Jones Salvage Yard owned by Jupiter's aunt and uncle. Headquarters is piled with junk that hid it from the curious. Only the Three Investigators know that, and they have a secret way for entering into it.

There are 43 books in the Original Series.