"He's only nine years old - but he is the world's most audacious special agent! There has never before been a special agent like JOE 90 - and television has never before presented such an original, imaginative and intriguing series."
"For nine-year-old Joe can do anything, thanks to a fabulous electronic device which can transfer the brain patterns of those who are the greatest experts in their field. When he receives these brain patterns, Joe, with the aid of a special pair of glasses which have built-in electrodes, becomes a man in thought and deed... can become the greatest of all astronauts, a dare-devil pilot, an ace motorist, a brilliant brain surgeon or whatever else may be necessary for him as the most daring of all agents attached to the World Intelligence Network... yet he still looks what he really is - a normal, healthy schoolboy. Every assignment brings new excitement, new hazards and unexpected situations - for JOE 90 and viewers alike!"

This was how the series was promoted in the original ITC publicity brochure.

Professor Ian "Mac" McClaine, a brilliant computer expert, has invented an incredible device called BIG RAT - Brain Impulse Galvanoscope Record And Transfer. It enables the knowledge and experience of one person to be transferred to another. He demonstrates the machine to his friend Sam Loover, by transferring his own brain patterns to his adopted son Joe.  Amazed, and recognising the machine's true potential, Sam temporarily dissuades Mac from selling the machine to Convex Computers. Professor McClaine meets with Sam's superior Shane Weston, head of the World Intelligence Network in London. Shane eventually persuades Mac to keep the BIG RAT a secret, and to enable W.I.N. to use it for missions - with Joe as a secret agent! After all, who could possibly suspect that a nine year old boy could be The Most Special Agent of W.I.N.?

But why was the series called Joe 90? Well you can choose from two theories. According to publicity information, when Joe joined W.I.N. there were 89 London based agents in operation. Joe became the 90th - hence JOE 90. But according to the episode Project 90, Joe's codename is due to the BIG RAT's designation as W.I.N.'s "Project 90". Take your pick.